I'll never acid wash my pool again! This is what I know and/or think I know about acid washing. You should research it
very carefully and then decide for yourself (as you are doing). It is not a trivial thing and, if warranted, should only be done with the utmost caution, by someone experienced. Acid washing works by removing a layer of your plaster, and thus the stains with it. It is not a magic liquid that removes only the stains and algae while leaving the plaster alone. Nope, it takes the plaster with it. Acid washing will reduce the life of your plaster, because it's removing a layer of it. Some estimate 1 to 3 years of life per acid wash, poof. There is a school of thought that you NEVER acid wash a pool. You live with the stains and save the money and replace the plaster when you can't go any longer. And another school of thought that you ONLY acid wash a pool as a very last resort, when the plaster is already on its last legs and the look of the plaster is so bad that an acid wash will make a slight improvement and get you an extra period of time before you have to replace the plaster anyway. The third school of thought is the most common (unfortunately, IMHO) shared by most pool maintenance companies who love to "recommend" an acid wash because it's easy to do, gets
some results and doesn't take very long, so it's a high-profit item for them. They don't care (and won't tell you) that the wash will cost you in plaster longevity.
Further... acid washing doesn't really remove calcium. It'll take some with the plaster, that's about it. Calcium is harder than plaster, and if you put enough acid on the surface to dissolve the calcium, you're going to take way more plaster with it. One plaster guy I talked to claimed they use a very diluted amount of acid to "soften" the pool, and then hand sand the calcium off. I think there's a ring of truth in that. If I had stains that I just couldn't live with, and was going to empty the pool, I'd hand-sand the stains. I'd be tempted to try a very diluted mixture of acid, to get things going, but I'd be prepared to neutralize the acid immediately to stop the reaction. And I'd do so knowing that that patch will not end up feeling as smooth as the rest of the pool (nor last as long). But again, only as a last resort. I'd spot sand
only, if possible, and that's it. Or use bleach. Or live with the stain.
Calcium around the water line can be blasted off, and you don't need to empty your pool for that. The blasting material falls to the pool floor and its vacuumed up.
My pool company recommended an acid wash, charged me $900 for it, and completely destroyed my plaster. I'm in the throws of legal action about it. You can read more about it
here if you care to. This is a small pic of the least they did, the damage at the bottom was way worse. I had to replace all my plaster.
Acid washing is done all the time. People do get results. My pool was an exception, for sure. But based on my experience and my pool, I think it's fair to say: an acid wash performed perfectly will give you some cosmetic improvement, reduce the life of your plaster, and leave your plaster feeling kind of, for lack of a better term, "etchy." Not rough like sand paper, but not new-plaster smooth. An acid wash performed less than perfect will take additional years off your plaster's life and could possibly destroy it all together (worse case scenario).
Acid washing an entire pool, as a maintenance procedure, that exhibits only small stains, is completely inappropriate.
It is my opinion that you shouldn't acid wash your pool (especially considering how nice it looks in your picture).
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Oops, just reread your question. If you're just talking about rust spots, there are chemicals you can apply that will attack the rust only and leave the plaster alone. Waaaay better than what acid would do. Something like this:
https://www.amazon.com/Super-Iron-Out-Stain-Remover-18/dp/B000BQRBS2
But try it on a small inconspicuous area first, to make sure it doesn't cause some other problem (something I wish my pool guys had done).