Acid Washing Pools
In areas where there are a lot of concrete swimming pools, acid washing pools is big business for service companies. For anywhere from a few hundred dollars, to a thousand or more, you can have your pool drained and acid washed and it definitely will look a lot cleaner and newer. The only problem with this is that it will be anything BUT newer.
When you use a pool plaster for the interior surface of your pool, it is not the plaster itself which provides an increase in water resistance, but the finish on the plaster. Plaster, just like every other type of concrete, is porous. When plaster is used as an interior surface in a concrete swimming pool, a technique known as a "hard trowel" is used which is essentially a final, very firm, pass with a steel trowel once the plaster is almost completely stiff. With a skilled hand this will result in an extremely smooth plaster finish. It is this smooth finish that increases the water retention abilities of the plaster - not just the plaster layer itself.
I recently was reading an article on acid washing on a popular "how to" website (which is read by millions of people) and it said that it is OK to acid wash every two years since pool plaster is 1/2" thick on average and acid washing only burns off the surface layer of the plaster. This is factually inaccurate and really a shame that so many people are going to damage their pools as a result of bad advice such as this. If the thickness of the plaster is the reason that plaster improves water retention in a swimming pool, then why do you need to redo plaster when it is still clearly there? The reason, is that once the smooth finish wears away on your pool plaster, you have effectively lost all improved water resistance over any other form of untreated and porous concrete.
Acid Strength For Acid Washing Pools
One of the biggest concerns about acid washing pool plaster is what strength of acid you should be using. What most pool service companies will do is use 4 to 1 or even 3 to 1 water to muriatic acid. This results in an acid wash that foams, spits and hisses when you pour it over the plaster. This is far too strong to use on pool plaster! When correctly done, an acid wash should be 90% acid brushes and elbow grease. This is contrary to the method that is used by that majority of service companies.
If you are applying acid wash to a pool plaster and the mix foams and hisses then you are very likely damaging the plaster. Sure the plaster will be brilliantly clean, but that bubbling and hissing is the acid eating the surface layer of the interior finish...the layer that is supposed to be smooth to help with waterproofing. Worse than this, some intrepid pool guys will use 50/50, or straight muriatic acid, which essentially will ruin your plaster right there on the spot regardless of how old it is. Most people do not understand that you judge the quality of the plaster on how it feels, and NOT how it looks. Acid washed plaster can look clean but if it feels like 50 or 100 grit sandpaper then that is a problem.
When you apply acid wash to a concrete pool there should be only the slightest hint of bubbling. In order to clean the plaster you need to very aggressively scrub the surface while the weak acid solution is on it. A small amount of phosphate free liquid dish soap added to the acid wash will help it to cling to horizontal surfaces longer while you scrub it with brushes. The amount of acid that you need will range between 8 to 12 parts water to 1 part acid. I would be willing to bet that almost none of the service companies reading this use acid this diluted...because it takes a long time, is super hard work, and does not look as clean as plaster that was blasted with surface-melting full strength, or near to it, acid.
Acid Washing Effect On Pools
If you need to get a pool clean, and fast, then acid washing is going to seem very appealing. What more concrete pool owners should know is that a pool plaster should only ever be acid washed once, perhaps twice, in the entire service life of the interior surface. Further to this, this means a mild acid wash as described above. If the acid is strong enough to bubble and hiss then it is literally eating the surface layer of the plaster - not just the stains and scale on top of it.
Plaster is made from essentially just sand and cement. If you want to learn more about pool plaster cement mixes you can find that information in the concrete basics section of this website. When plaster ages, the portland cement within the plaster will dissolve and wash away with the water leaving only the aggregate behind - sand. This is why plaster that is old has a very rough, very sandy texture and feel. When you acid wash a pool you are accelerating this process by years in a matter of minutes. You can take a brand new plaster and acid wash it and essentially ruin it completely. Once rough and sandy, with all of the cement washed away, pool plaster is as porous as every other kind of concrete. As a pool owner you would not immediately notice this. There is no other sign other than the plaster feels rougher than before, but in the long run your pool will lose more water, use more chemicals, be harder to keep clean and clear, and likely develop other long term deficiencies to the shell and structure.
So why do people acid wash pools at all? There are cases where a very mild acid wash is the lesser of two evils when it comes to cleaning severely neglected pools. When a pool owner thinks that they need an acid wash, what they probably need is a new plaster surface altogether...good luck selling that to a customer when the next pool guy to come along says he can make the pool shine again for under a thousand dollars. This other pool guy burns the hell out of the plaster, but it looks good so the customer is happy. The plaster was probably already rough from age, which is why it was staining to begin with, but to the customer the pool looks new again and they don't notice it being any rougher than before. This is how you take something that should last 100 years and make it basically fall apart in under 25. The interior surface in a concrete pool is one of the single most important components of this type of pool, and operating your pool for years with little to no water resistance will be sure to add up to early structural pool failures and expensive repairs.
Pools With Colored Plaster
Any concrete pool builder knows the challenge of selling plaster with colored aggregate in it. Colored aggregate is a fairly expensive cost upgrade over white or charcoal colored pool plasters. White silica sand and fine graded dolomite is much cheaper as an aggregate than aqua colored crushed crystal dust. If a swimming pool owner wants to have a fancy colored interior surface then it will end up costing them more...but that is not the problem.
The real problem with colored aggregate in pool plaster is that once the plaster is complete there will be a cement haze left over the entire surface of the pool. This is completely normal and is not an indication of a problem with the plaster installation. Still, there are many new pool owners that get upset since the colored plaster that they paid extra for basically looks terrible. The solution to this for many builders, unfortunately, is to acid wash.
Acid washing pool plaster is not a great idea for the plaster to begin with. Acid washing brand new plaster that has not even fully cured yet is a sure way to cut the service life of your plaster in half - or worse. The solution to dealing with this problem with colored plaster should happen during the selling stage. Pool owners should be made aware of the fact that concrete haze is normal on a new pool plaster, and is not a deficiency, and a strong acid wash will make it look instantly better...but at a huge cost. Under normal conditions it should take between one to three years for a colored plaster to achieve maximum color effect as this is how long it will take to wear away the surface layer of cement on the plaster. Should an acid wash be done early in the life of a plaster in order to get the maximum color effect for a customer from day one, a very weak acid solution around 10 to 1 should be used along with extensive brushing of the surface. If the acid is doing the work for you then the mix is too strong.
How To Acid Wash A Pool
It is unlikely that this one article is going to change the way that service companies approach making money and acid washing swimming pools. The vast majority of information on the internet talks about 25% muriatic acid being applied to pool plaster every few years. As a concrete pool expert this absolutely drives me crazy since this information will lead to thousands of pools failing early and "nobody" to blame. The correct answer to how to acid wash a pool is to first explore every other potential option before acid washing, then, assuming you accept the risk and will give up a little life from your plaster to have the pool look cleaner, then acid wash with a very diluted acid mix and make the process more about the manual brushing and less about the acid etching and burning off the cement in the surface of the plaster.
Acid Magic is an option that some pool companies use for acid washing pools. Acid Magic is a muriatic acid alternative that is often confused with simply a weak or diluted acid. Acid Magic is actually a full strength acid alternative and it is not weaker than muriatic acid - it is actually a stronger acid than muriatic acid but it does not burn exposed skin in the same way that muriatic acid would due to proprietary buffering agents. It is actually safe enough for use as a drinking water treatment but at 4 to 1 mix strength as the manufacturer suggests, this is still too strong to be safely used on plaster.
Acid is cheap, and applying some to an old and stained pool plaster will certainly make it look new, but not everything that glitters is gold. This is just one more reason that the pool and spa industry needs to establish and enforce a more comprehensive congruency of technical process. This along with trade licenses that require years of training and experience to get just like auto mechanics, carpenters and electricians are sorely needed. If this is interesting to you then you can read more about this in this article on trade certification in the pool industry.
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