The concept of the “Zero Alkalinity” process (also known as a No Drain Acid Wash or Bath) is to make pool water aggressive enough (by adding acid) to dissolve and remove stains or scale from older cement-based plaster surfaces, including quartz and pebble swimming pools. Unfortunately, this process generally removes some of the plaster surface material.
Assuming a total alkalinity of 100 ppm, it requires 4 gallons of muriatic acid (31.45% strength) per 20,000 gallons of pool water to reduce to zero alkalinity, and to a pH of 4.5. That acid treatment over a 7 day period can dissolve and remove about 8 pounds of plaster material (equivalent to 50 ppm of calcium carbonate) from a fully cured surface. So is that good or bad for a plaster pool?
The reality is that 8 pounds of plaster material dissolved from a 20,000 gallon pool is not significant and would barely be seen under magnification. If stains are removed that improve the aesthetic appearance of older pools, then that can be an acceptable program. If the acid process only removes dirt and scale deposits and not calcium from the plaster itself, well, all-the-better.
Some new white plaster pools develop a blotchy and darkened gray mottling discoloration within a few months of plastering. Also, some new dark colored plaster pools develop a whitish and streaky discoloration soon after plastering.
Those issues are often caused by improper plastering practices. The chemical start-up program (performed by the pool owner or service tech) may not be at issue, but is often (incorrectly) blamed by the pool builder or plasterer. A no drain acid treatment generally does not properly resolve these type of plaster discoloration defects. Sometimes, the discoloration disappears for a few weeks, but soon returns afterwards.
Unfortunately, representatives of the National Plasterers Council (NPC) often promote the Zero Alkalinity (No Drain Acid Bath) program as a “solution” for the above problems. But making matters worse, they advise adding 10 gallons of acid to 20,000 gallons of pool water, more than double the amount needed to reduce the alkalinity to zero ppm. That excessive amount of acid added can lower the pH to below 3.5, lower the LSI to a negative (-) 5.0, and dissolve about 20 pounds of plaster material from the surface. (Twenty pounds of plaster material is equivalent to 120 ppm of calcium carbonate dissolved from a plaster surface which then increases the calcium hardness of the pool water).
And if adding all that acid doesn’t improve the appearance of the plaster well enough, amazingly, they advise to double the dose of acid for another week! It removes about 40 pounds of plaster material! That treatment is absolutely detrimental to the plaster surface, and a disservice to the pool owner. It even destroys the evidence of the real cause of the plaster discoloration defect!
That type of acid treatment causes an severely etched plaster surface that was originally smooth, which will then begin to stain easier and sooner by dirt and metals in the water after the acid treatment program has been performed. When the pool becomes stained and aged within a year after a Zero Alkalinity treatment, who is going to be blamed? Unfortunately, whoever is maintaining the pool water chemistry, not the plasterer.
To further undermine pool owners and service techs, the plaster industry advocates that pool water with a slightly negative LSI (-0.1 to -0.3, which is within APSP standards), is detrimental and that it will cause everything from craze cracking to gray mottling to plaster spalling (delamination). Yes, even the same gray discoloration that the so-called Zero Alkalinity process attempts to remove. Obviously, that is contradictory and completely false. No wonder they claim that pool plaster will only last about 7 years. But the fact is plaster will last 20 years if plastered with good workmanship and maintained properly.
That agenda enables plasterers avoid being held responsible, blame water chemistry, and get rewarded with additional work. This injustice won’t stop until pool builders and other groups confront the plasterers.
("No Drain Acid Wash" is a trademark of United Chemical Corporation).