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Plaster Cracks Explained

From The National Plaster Councils FAX

Cracking has occurred in my in ground swimming pool’s plaster. Why did this happen and how can it be remedied?

There is more than one reason why an interior finish could be cracking. First, you need to diagnose which type of cracking has occurred. Closed shrinkage cracks, open shrinkage cracks, and structural movement cracks are each unique, in terms of the cause and remedy. Each type of cracking is defined in the NPC 9thEdition Technical Manual. (Some sections that deal with cracking are given below)

New installations and drained pools: A common appearance of cracking can occur when a cementitious interior finish is wetted prior to being filled. Microscopic shrinkage cracks (micro cracks are present in

all hardened plaster, precast, shotcrete, and concrete products) absorb water faster than the overall surface. This results in these micro-cracks becoming visible. The cracks (having the appearance of a

road map) appear to be ‘highlighted’ by the water absorbing into them. Once the water evaporates, or once the pool is filled with water, they are no longer visible. In fact, these micro-cracks actually seal themselves shut with cement hydration material after the pool is full. These cracks are known as ‘check cracks’ (or closed shrinkage cracks). They are common to all cement and concrete products, and are generally nothing to worry about. However, any cracks that remain open 24-hours after the pool has been filled should be reported to the Pool Builder or Plastering Company.                  

The following types of cracking are generally in need of some type of repair. 

NPC Technical Manual

Cracks in cementitious surface coatings should be classified as to their type, prior to determining the optimum method of repair. Cracks can be classified into one of these three types:

– Closed shrinkage cracks that develop during application or within the first twenty-four hours after final set. 

– Open shrinkage cracks that develop during application or within the first twenty-four hours after final set.

– Structural movement cracks, tensile stress cracks, and all other cracks, that develop throughout the life of the coating, subsequent to the first twenty-four hours after application.

Closed Shrinkage Cracks

Closed shrinkage cracks, or normal shrinkage cracks, form from the normal volume loss that occurs within a fresh cementitious surface coating, due in most part, to moisture loss that the cementitious surface experiences as it sets and dries. Shrinkage-related cracks are typically reversible, meaning they re-seal themselves and therefore are generally considered a cosmetic problem. These cracks tend to fill or seal shut underwater within (24) twenty-four hours after total immersion in water. As the cement hydration continues, some of the newly formed compounds (primarily calcium hydroxide) fill the crack space. For many other cementitious products this may not be the case, however, for cementitious materials that are kept in total submersion in water, shrinkage-related cracks will generally reseal. A crack that remains open after immersion would be classified as an open shrinkage crack.

Upon draining of a swimming pool, closed cracks may become visible when the dry cementitious surface is initially wetted. Though the crack is sealed or closed, the density of the material within the crack is lower than the overall density of the remainder of the surface coating, therefore, water will absorb into the closed crack areas at a faster rate than the remainder of the coating. This allows the once invisible cracks to become visible, for a time, until either the plaster coating becomes fully saturated, or fully dries.

Closed cracks are more noticeable on smooth surface finishes. Closed cracks can also become noticeably visible if, prior to their sealing, dirt or dust becomes trapped within the cracks. The cracks re-seal underwater, however, the location of the cracks remains visible, due to the dirt trapped within. Check cracks do not affect the performance of the surface coating and are considered a normal characteristic of the cementitious surface coating.


If the cracks do no reseal, refer to “Open Shrinkage Cracks” for a remedy. If dirt is trapped within a sealed check crack, two options are available as a remedy:

– Closed shrinkage-related cracks can be opened with a knife, an abrasive grinding blade or similar tool, allowing the cementitious mixture to penetrate, or key into, the crack.

– Closed shrinkage-related cracks can be left alone. Check cracks generally only affect the upper surface of the cementitious coating and do not penetrate through the cementitious surface coating. Check cracks seal underwater, rendering the check cracks harmless. Isolated small check cracks can also be left alone in terms of their effect on the intended functional purpose of the cementitious surface.

The decision to repair a shrinkage-related crack that is closed, yet remains visible, is generally for aesthetic concern only. Determining what is considered “acceptable” in this case can be problematic. Arbitration can sometimes be the best option (see Section 1.8 of the Tech Manual). Shrinkage-related cracks that are closed, yet remain visible due to a color variation from that of the remainder of the plaster coating, can sometimes be drained and allowed to dry and/or bleached to remove possible trapped moisture and to even the colorations across the plaster surface.

Open Shrinkage Cracks

Open shrinkage-related cracks are formed when excessive moisture loss creates stress cracking within the surface coating during set or drying that do not reseal. Open shrinkage cracks can be cracks that penetrate only the upper 1/8” of the surface of the coating, or they can be cracks that penetrate through one of the coats of the surface coating. The following are known causes of open shrinkage cracking:

– The water loss from the surface coating during the plaster application due to the evaporation of moisture from the upper surface (insufficient supplemental “re-tempering” water).

– The water loss from the surface coating during the plaster application due to the absorption of moisture from the substrate surface (insufficient supplemental “tempering” water).

– The water loss from the surface coating during application due to extreme weather conditions, such as; high wind, high temperature, and/or low humidity.

– The loss of moisture from the surface of the coating due to the high absorption of the substrate, such as; a substrate that is placed using a non-homogeneous mixture; an insufficiently wetted substrate to start the absorption process just prior to surface coating application; an overly-dry substrate that has not been soaked down to lower its’ absorptive capacity; or improperly cured substrate.

– The use of excessive supplemental (temper) mix water, beyond that amount needed to maintain a proper time of set and/or the early ongoing hydration (over-tempering).

– The use of excessive lubricating water, beyond that amount needed to ensure proper placing, tooling, and/or finishing of the cementitious surface.

– Working the surface while the coating is too wet or working the finish too much (over working).

– Not sufficiently working the surface coating at the appropriate time when the tensile strength of the surface coating exceeds the stress associated with initial water loss. In other words, the entire thickness of the coat is too weak to withstand the water loss, creating plastic shrinkage (under-working).

– A cement-rich mixture, beyond the normal standard of the trade as conforming to the NPC Technical Manual and/or ASTM C 926.

– The neglected or delayed filling of the swimming pool with water immediately following the completion of the plastering application.

– Some combination of the above.


Smaller open shrinkage-related cracks should be cleaned and filled with a cement paste, with or without a polymer additive; or with cement and a fine sand, with or without a polymer additive or other patching material. The repair material should be forced into the crack and the excess material should then be rubbed off with a glove or a fabric using a motion across and not along the crack. Larger open shrinkage-related cracks should be repaired in a similar manner, except the crack patching material should have a sand or a filler material with a slightly courser grit included.  Surface coatings having open shrinkage cracks that are too numerous, or in situations where numerous crack-patch repairs would not be considered acceptable, resurfacing may be the only alternative. Determining what is considered “acceptable” can be problematic. Arbitration can sometimes be the best option. (see Section 1.8)

Structural Movement Cracks or Tensile Stress Cracks

Structural movement cracks or tensile stress cracks are significant cracks that extend through the entire thickness of the cementitious surface. Unlike shrinkage­ related cracks that are limited to a maximum crack width as defined by the volume change that a cementitious surface undergoes during its set, structural movement cracks can continue growing in length and width until movement stops in the underlying structure or the cementitious surface. Structural movement cracks typically follow the stress pattern that the crack relieved. Structural movement cracks are generally found at areas of stress, and typically not in numerous numbers, as compared to shrinkage-related cracks.

Structural movement cracks can be working cracks or non-working cracks. Working cracks are the result of an ongoing structural movement that continually opens, closes, slides, lifts, or lowers the cementitious surface at, or along, areas of stress in the pool structure. Non-working cracks are inactive and do not exhibit further structural movement. Working and non-working structural movement cracks can be repaired, however, in order to repair certain working cracks, the cementitious surface should be removed and the structural problem causing the movement corrected. If the cracks are non-working and the stress issues creating the original structural movement has stopped, then the cementitious surface can be repaired in a similar fashion to that of open shrinkage cracks (see Section

Possible contributors to the cause of structural movement cracks or tensile stress cracks can be:

– Structural movement of the swimming pool structure due to soil expansion or settlement.

– Improper structural design of the swimming pool, allowing movement or cracking of the substrate and thereby cracking the surface coating.


Smaller structural movement cracks or tensile stress cracks should be cleaned and filled with cement paste, with or without a polymer additive, or cement having a fine sand, with or without a polymer additive, or other appropriate patching material. Larger structural movement cracks or tensile stress cracks should be cleaned and filled with cement having slightly coarser sand, with or without a polymer additive, or other appropriate sanded patching material. The repair material should be forced into the crack, and the excess material should then be rubbed off with a glove or a fabric using a motion across, and not along the crack.

The remedy and repairs for structural movement cracks, as described herein, are either for cosmetic repair, or for a temporary patch repair, to the surface coating only. These cosmetic repairs do not take the place of any structural repair that is necessary or required to restore the swimming pool structure.

There are white spots/blotches in my pool’s plaster, why did this happen and how can I remedy it?

One of the most common issues in pool plaster is etching deterioration, or ‘spot etching.’ This can result from aggressive water chemistry: low carbonate alkalinity and elevated cyanuric acid are the main contributors to spot etching. You can calculate how to keep your water chemistry in proper balance using the Langelier Saturation Index (LSI), as seen on the NPC Start-Up Procedures Card, which is offered here. Section of the NPC 8th Edition Technical Manual explains in detail what an “aggressive chemical attack” is and what it means for your plaster’s health.

Aggressive Chemical Attack

Cementitious surface coatings are susceptible to chemically aggressive swimming pool water or to any chemical that is capable of deteriorating the cementitious materials of a surface coating. Swimming pool water that is not in Ideal Range of acceptable tolerance (or “balanced”) as in accordance with the ANSI/APSP Standards is considered to be detrimental to cementitious surface coatings. Depending on the direction of the imbalance a coating can either be stained or etched. Often, both staining and etching are exhibited on the surface of a coating as the pool water fluctuates from one extreme to the other. Visible signs of salt crystallization, surface efflorescence, or cracks that are emitting efflorescence or salt deposits are common to a coating in contact with a positive saturation index (+ LSI), or “scaling” condition. If the water is capable of leaching cement compounds from within the surface coating, then that water is in a negative saturation index (LSI), or “aggressive” condition.

If care is not taken to ensure that the water of a swimming pool is kept in an Ideal Range that is considered by ANSI/APSP to be balanced, and further that a sufficient amount of carbonate alkalinity buffer is constantly present, then the cementitious surface coating should be considered susceptible to damage from that water and deterioration may result. Water-soluble salts, acids, or ions such as sulfates, chlorides, and carbonates, can be transported into a cementitious surface coating and react with the cementitious compounds. The resulting chemical reaction between the cementitious compounds and these salts, acids, or ions, in the presence of water, can cause deterioration to the coating. Acidic water aggressively attacks and dissolves cementitious surface materials. Water that is low in hardness, or soft water, can cause leaching of certain compounds of the cement, especially calcium  hydroxide.

The National Plasterers Council’s technical experts have covered this issue in great detail on YouTube, which you can watch by clicking here. The NPC encourages you to watch these videos and take the time to educate yourself in order to better maintain your pool’s plaster.

My pool is being plastered/re-plastered. What are the industry standards and/or guidelines and methods for plastering a pool?

The NPC Technical Manual includes industry guidelines and recommendations that are based on the current-accepted trade practice for swimming pool plastering in the United States. The NPC Technical Manual is recognized and referred to by several organizations, such as ACI, APSP, PCA, ASA, ICC, and ANSI, as well as many regional Health Departments, and  Local, State, and Federal Agencies. The document is periodically canvased, reviewed, and revised to update and incorporate the latest information on common trade practice. The Technical Manual includes a comprehensive overview of materials and application for swimming pool plastering, to include: Aesthetics and Tolerances;, Materials Guidelines;, Surface Preparations for Pool Substrates;, Common Trade Practices – Mixing and Finishing Techniques;, and General Performance Guidelines. This great resource of expert-driven information can be found on the NPC’s website here.

A more detailed listing of the topics covered includes:

Aesthetics and Tolerances: general application, water-tightness, smoothness, texture, levelness, thickness, thickness exceptions, maintainability and service life, and arbitration.

Materials Guidelines: batching and mixing, mix design ratios, field water-to-cement ratio, cementitious materials, sand/aggregate, water, admixtures, calcium chloride, polymeric admixtures, pozzolanic admixtures, and pigments.

Surface Preparations for Pool Substrates: surface conditions for new pool substrates, surface conditions for renovation pool substrates, surface-applied bonding agents, integrally-mixed bonding agents.

Common Trade Practices – Mixing and Finishing Techniques: proper usage of mix water (Temper water) and supplemental mix water (Retemper water), proper usage of lubricating water.

General Performance Guidelines for Exposed-Aggregate Finishes: exposed finish types, exposed-aggregate finish, exposed-quartz finish, polished finish, general characteristics, water-tightness, over-exposed finishes, under-exposed finishes, determining exposure tolerances.

There are small-to-large ‘bumps’ protruding from my pool’s plaster and/or tiles. What are these and what can be done to remedy the issue?

Calcium carbonate formations on the surface of a finish coat are generally the result of the percolation of water that is capable of dissolving or leaching cement compounds or calcium from voids, bond failures and cracks, which is then precipitated onto the surface. These are known throughout the industry by the slang terms “nodules” or “calcium nodules.” The actual mechanism and source of the precipitated calcium formations can be from:

1.) a failure to achieve an adequate bond between the interior finish coating and the substrate surface, or

2.) a prolonged ‘aggressive’ pool water that attacks the cement compounds just below the finish surface (the sub-boundary zone). These YouTube videos help to explain the cause of these formations and what can be done to remedy the issue.

What is the difference between ‘debonding’ and ‘delamination’ ?

Debonding is an “adhesive” bond failure, or the failure of the interior finish coating to bond to the cementitious substrate.

Delamination is a “cohesive” bond failure, or the failure of the interior finish coat to hold together, or to split apart.

Debonding failures generally occur during the original placement of the interior finish coating, from a failure to achieve an adequate bond with the substrate surface

Further information can be found within the NPC Technical Manual (a portion of which is given below):

A coating that has achieved a proper mechanical bond will generally maintain that bond throughout the service life of the coating. A physical bond, or a mechanical bond, is created when the plasterer forces the wet coating material into the open, porous, scratched, etched, or otherwise roughened substrate or undercoat, thereby keying into, or interlocking the two surfaces. The failure of a cementitious surface coating to properly bond with the solid substrate or cementitious surface undercoat is generally due to one of more of the following:

– Surface carbonation of the undercoat or solid substrate increases the surface density, or decreases the porosity of that surface, which can reduce or prevent the ability of a surface coating to achieve proper bond.

– Surface efflorescence or surface laitance on the substrate or undercoat can reduce or prevent the ability of a surface coating to achieve proper bond.

– An overly smooth or overly dense substrate surface or undercoat surface can inhibit a surface coating from keying into, or bonding with, the substrate or undercoat.

– An overly dry substrate or undercoat, or a substrate or undercoat having a high rate of absorption. A rapid moisture loss from the coating at the interface of the coating and the substrate can cause the coating at this interface to stiffen quickly. Rapid drying at this interface, as opposed to a slow stiffening or setting can cause debonding. This scenario leaves the plasterer with little or no time to force the wet coating material into the pores of the substrate before rapid drying occurs.

– A solid substrate or undercoat that is overly wetted, or overly water­ saturated, prior to receiving the surface coating. Water or moisture that is visible on or within a substrate that occupies the open pore spaces, restricts the ability of the coating to key into the substrate, as well as, reduces the absorption capability of the substrate. This can prevent or reduce the proper bond of the surface coating to the substrate.

– A coat that is applied too thinly over the substrate can dry before it sets. Thin coats can have excessive and/or tensile cracking within the coating caused by the normal stresses of shrinkage far exceeding the low tensile strength of the thin coat. The result is a weak cementitious surface that may not maintain bond.

– A surface coat that is applied overly thick. The coating can exhibit abnormally high shrinkage resulting in the structural cracking of the coating and possible debonding.

– Contaminated substrate surfaces can reduce or prevent bond. Examples of contaminants would be dirt, oil, acid, form-release agents, or loose debris.

– A scratch-coat, brown-coat, or other surface undercoat that is not properly scratched, scored, notched, or otherwise properly prepared to receive the surface coating.

– Delamination within the upper nominal 1/8” (3.2mm) of the surface coating can be caused by over­ troweling the finish surface of a coating, or by troweling the finish surface of the coating beyond the time the coating is fully hardened. Failure to immerse the surface coating placement underwater after coating underwater after placement.

– Failure to maintain immersion of the surface coating underwater throughout the service life.

– Failure to maintain constant moisture to the surface coating in the rare event that a swimming pool is drained.

Buckles, Blisters and Spalls

Buckles are bulges of the surface coating due to a separation from the substrate, or from the undercoat (debonding), or from a delamination within the coating that have not yet lifted off or broken away. Buckles are generally caused by a failure of one coat to bond to another coat, or the failure of the surface coating to bond to the substrate. (see Section 7.4.1) Buckles are caused by the same issues as delamination or debonding.

Blisters are small round bumps on the finish of a surface coating. Blisters are separations occurring within the top coat or finish coat that have not yet lifted off or broken away. Generally, blisters are caused by trapped air during the finishing process, over-troweling, or troweling beyond final set.

Spalls are upper-layer delaminations or separations occurring within the top coat or finish coat that have lifted off or broken away. Spalls can be caused by abrasion or hard blows to the plaster surface, aggressive water chemistry, or from the plaster not being submerged in water, and/or being dry for too long.

Blisters and spalls are generally delaminations of the thin upper-finish layer of the surface coating.

Blistering and spalling can be caused from the following:

– Over-troweling of the finish.

– Troweling past final set.

– Failure to immerse the surface coating underwater after placement.

– Failure to maintain immersion of the surface coating underwater throughout the surface life.

– Failure to maintain constant moisture within the surface coating in the rare event that a swimming pool is drained.

– Repetitive impact or abrasion to the surface coating during service life.

– Overly-aggressive swimming pool water chemistry. Over time, aggressive water will attack and deteriorate cementitious products. The upper-finish layer of the surface coating (approximately 1/8” [3.2 mm] in depth) is especially affected. Calcium hydroxide, the weakest hydration product of the cementitious surface coating that can be leached from this layer causing this upper layer to weaken and/or detach from the remainder of the top coat.2,7,9,11,16,17,22,23,24

– Freezing temperatures that occur after final set but prior to submersion.


Isolated debonded areas should be removed. The surface that is to be patched should be clean and made free of all loose materials or contaminants. A bonding agent should be used prior to, or in conjunction with, the application of the repair coatings. The new coats should be applied in a similar manner, and in the same number of coats, as the existing cementitious surface. It is considered common practice to remove completely, the portion in need of repair down to the substrate, thereby ensuring that the new coating is properly keyed into the substrate and the remaining coating. The repair coating should be properly cured. The entire coating can be removed and replaced, re-coated or multi-coated, stained, painted, or otherwise treated if the repairs are too numerous or overly noticeable to the point of being unacceptable.

Small blisters and spalls, lifted areas, broken or chipped areas, and minor blemishes can sometimes be repaired underwater. Buckles, blisters, and spalls that have not yet lifted off or broken away can be left alone without fear of damage to the structural integrity of the swimming pool; however, any visible cracks should be filled underwater to prevent water from freely entering into the void space and leaching cementitious compounds or other coating materials.

What is the ideal range that I should be maintaining my pool’s water chemistry within?

Note: The NPC’s recommendations for ideal water chemistry are similar to those of the APSP. Some high-performance finish coatings may recommend slightly different parameters. While the NPC recommends water chemistry ranges based on trade and industry consensus, it is recommended to verify with the Builder, Plasterer, or Product Manufacturer as to the specific ranges they require. Contractual agreements supersede NPC recommendations. 

Water chemistry for your pool varies depending on whether or not the plaster is within the first 28 days of its life. After the 28 Day Start-Up Procedure has been performed, daily water chemistry should be maintained within the ‘ideal range.’ This range is as follows:
                – Free Chlorine = 1 to 3 ppm
                – Total Chlorine = 1 to 3 ppm
                – Sequestering Agent as per Manufacturer’s directions
                – pH = 7.2 to 7.6
                – Carbonate Alkalinity = 80 to 120 ppm
                – Calcium Hardness = 200 to 400 ppm
                – Cyanuric Acid = 30 to 50 ppm
                – TDS = 300 to 1800 ppm (non-salt pools)
                – Salt Level = according to the manufacturer recommendations (salt chlorination ONLY)

Using the LSI Calculator (Langelier Saturation Index) you can determine if your water chemistry is balanced within the spectrum of |Scaling — Corrosive|. The LSI of your pool’s water should be 0.0 to +0.3. You can find the LSI Calculator on the NPC Start-Up Procedures Card here.

I live in an area where I am required to ‘winterize’ my pool. Is there anything I need to know in order to preserve my pool plaster’s health during the off-months?

In many regions of the country, freezing and thawing (freeze/thaw) damage of a swimming pool surface coating is rare, as the coating remains underwater during its lifetime. However, any portion of the interior finish coating that is not constantly underwater is susceptible to freeze/thaw deterioration and damage.

Cementitious finish coatings are most susceptible to freeze/thaw deterioration when a portion of the coating is partly submersed in water and a portion is partly open to the air. Pool finish coatings are not capable of withstanding the internal expansion forces created under these conditions. In the presence of moisture and intermittent cycles of freezing temperatures, the portion of the surface coating that is within 3” above of the water level, as well as the portion that is within 1” – 2” below the water level, are very susceptible to severe deterioration and damage. The deterioration will initially present itself as a horizontal line or area of discoloration near the water level, and often a heavy calcium salt deposit is present in the same area, as the cement binder is compromised. The deterioration will progress with future freeze/thaw cycles, and as the internal expansion and contraction destroys the cement binder system, causing a weak upper-surface. This is followed by peeling, delaminating, and eventually the complete failure of the coating. Most warranties do not cover freeze/thaw damage.    

It is recommended, for the durability and long-term service life of the pool interior finish coating, that it remain underwater. Therefore, winterizing methods that do not include the draining of a portion of the pool’s water are preferred. Other methods of winterization, which prefer the protection of the plumbing, equipment, and tile at the expense of the interior finish coating, often result in unsightly damage and a dramatic reduction on the finish coating’s lifespan.

How soon after my pool has been plastered or re-plastered can I swim in it?

At no time should any person or pets be allowed in the pool during the fill. Do not allow any external sources of water to enter the pool to help prevent streaking. It is recommended that you do not swim in the pool until the water is properly balanced, sanitized and the there is no more loose plaster dust from brushing; however, entering the pool once it is filled with water will not harm the finish.

When I turn the light on in my new pool I see lumps and waviness in the floor of the pool, is this normal?

Plaster and all cementitious interior pool finishes are a hand crafted product. In section 3.1 of the National Plasterer Council Manual 8th edition it states – General Tolerance Determinations: “Observation, using the swimming pool light at night, or other sources of light that shine across the surface, instead of upon the surface, are not considered a fair representation of the surfaces true appearance. Certain angles of light will grossly exaggerate imperfections and/or the waviness across the surface finish.” As it says some fluctuations and waviness in the pool surfaces are normal. If you cannot observe them in normal daylight viewing conditions it is probably normal.

I have an older pool that has 20 year old plaster in it. Are there better surfaces that have come to the market?

Yes- surfaces such as quartz, mini pebble, and pebble and polished surfaces are now available throughout the United States. Many of these products also have special modifiers in them such as pozzolans or polymers which enhance the long term durability of these surfaces. You should ask your remodeler about these types of surfaces. 

My plastering company recently advised me that a problem in my pool surface was attributed to “low carbonate alkalinity.” What is “Carbonate Alkalinity?”

Carbonate alkalinity is also known as corrected or adjusted alkalinity. The three most important water chemistry parameters necessary to have “balanced” water are A) pH, B) Carbonate Alkalinity and C) Calcium Hardness in their correct ranges. Most test kits test total alkalinity, but carbonate alkalinity is calculated by taking approximately one third of your stabilizer or cyanuric acid content and subtracting it from your tested alkalinity. Remember, only carbonate alkalinity can be used to calculate the Saturation Index and determine whether your water is in balance.

My pool plaster is approximately two years old. For the first 18 months the white plaster looked fine, but now is slowly turning blue, blue/green and black. What is this?

You are probably experiencing some type of metal stains. The most common metal stains are copper and iron.  Take a piece of #80 wet/dry sand paper or a diamond hand pad, and see if the stains will sand off. The four most likely sources of copper metal are 1) fill water, especially well water or trucked in water. 2) Improperly maintained pool water that becomes corrosive and attacks heat exchangers and other metal components. 3) Certain ionization systems that are not maintained properly and contain copper. 4) Copper based algaecides that are improperly dosed, most often to kill algae. The most common sources for iron include 1) metal pipes 2) fill water, especially well water or trucked in water 3) iron-based fertilizer accidently broadcast into the water, or by entering the water through wind and rain from surrounding landscaping

My finish was fine when initially installed, but I’ve begun to notice small brownish or rust colored spots.  What are these spots?

Based on your description these brown spots may be a) small metal particles from nearby work, such as the installation of wrought iron fencing or other metal work b) fertilizer stains from an iron or ammonium sulfate type granular fertilizer. c) some impurity within the finish. Usually these small spots will sand off with a piece of wet/dry sand paper or a diamond hand pad. If the spots do not sand off, there may be some impurity within the material, which will need to be treated chemically or picked out and patched by the builder or installer.

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