Kim – Ten Steps to Durable and Discoloration-Free Plaster

There are proper steps to follow for the making of durable pool plaster. There are also improper practices
that can lead to early deterioration, discoloration or other failures. Following is a ten-point checklist that will
help achieve a lasting and discoloration-free plaster.
1. The best cement/aggregate ratio is about one-part cement to 1.5-1.75 parts aggregate (marble sand). If
the plaster is too rich (cement-heavy), it tends to shrink and crack. If it’s too lean (more sand), it will be less
durable and potentially unworkable.
Note: Always select high-quality, consistent and appropriate-grade cement and aggregate, and non-organic
color pigments (not organic) for colored plaster.
2. When mixing plaster, aim for a water/cement ratio of .48 or less. Both the American Concrete Institute
(ACI) and the Portland Cement Association (PCA) maintain that lower water/cement ratios produce better-
quality cement that can better withstand occasional exposure to mild acids.
Lower water/cement ratios boost density while reducing permeability, porosity, shrinkage (craze cracking)
and water movement within the cement product. Adding too much water, by contrast, increases shrinkage
issues and fail to provide adequate protection or long-term durability against the effects of water and the
environment.
3. A plaster mix should be mixed thoroughly, but also not too long. It is recommended that if the plaster has
been mixed for more than 90 minutes, the plaster mix should be discarded.
4. Plaster should contain as little calcium chloride set-accelerant as possible – and never more than 2
percent to the amount of white cement. (Colored plaster, of course, should not contain any calcium chloride.)
According to the PCA, too much calcium chloride increases the potential for gray mottling discoloration and
cement shrinkage. Several alternatives to calcium chloride that do not exhibit these characteristics are now
available.
5. Never add water to plaster surfaces while troweling. Both the ACI and PCA have found that this may
increase porosity, shrinkage, and variable discoloration. A little water to lubricate the trowel, however, likely
won’t harm the plaster surface.
Still, you never want to “work,” or force, additional water into the plaster surface when troweling. Doing so
can weaken the surface and may accelerate deterioration and cause white spotting and streaking
discoloration. Colored plaster is even more susceptible than white plaster to discoloration from too much
water.
6. Well-timed hard troweling can help produce a nice, dense plaster finish. But if the plaster becomes too
hard before you have a smooth surface, late hard troweling often results in discoloration, mottling and
spotting, especially when calcium chloride is also used.
7. Plastering in extreme weather conditions can lead to quality and durability problems. Industry groups
specifically warn against using cement-based products in temperatures considered too hot or cold.
One solution is “tenting” the pool, which protects the plaster surface (and the plasterers!) from the elements.
In extreme heat, tenting the pool, and perhaps even directing air from an evaporative cooler beneath the
tent, will help the plaster retain its moisture, and properly cure and harden.
8. Don’t fill the pool with water too soon. Though conditions vary, water usually should not be added for at
least 4 to 6 hours after the pool has been plastered and finished. This should be enough time for the plaster
to harden properly before being submerged in water.
Even balanced tap water can dissolve certain plaster components if the surface has not adequately
hardened. The end result is often greater porosity and early deterioration. And it may only take a few months
for issues to show up.
9. Soft or aggressive fill water can also deteriorate new plaster surfaces, which manifests as a uniform
roughening of the surface paste. Other new plaster problems such as drips, splotches, spotting, trowel
marks, gray mottling, and footprints are the result of localized finishing errors.
Surfaces may be further damaged by aggressive (acidic) startup techniques, which can cause additional,
generally uniform (depending on the method of acid addition) surface loss. By contrast, bicarb startups can
neutralize aggressive fill water while promoting a superior plaster surface.
10. Once the pool is filled, balance the water (and keep it balanced). Balanced water helps help preserve the
plaster. Aggressive water causes uniform etching, while over-saturated water scales (deposits calcium
carbonate on) plaster. The Saturation Index is a good guide – to prevent scaling or etching, water should
have a saturation index value in the range of -0.3 to +0.5.
With reasonably consistent maintenance, standard plaster has a life span of approximately 20 years. It’s an
inherently strong surface and should be able to withstand “real world” water chemistry and/or maintenance
challenges.
Though pozzolans, blended cements and other materials are generating good results, there’s still no
substitute for solid workmanship. The above guidelines will benefit pool plasterers in the pursuit of a quality,
long-lasting, and discoloration-free pool finish.
This link is to an article that provides a step-by-step program on what to watch for during the
plastering of a pool to ensure a quality finish: