Pb Plans To Use Muratic Acid To Clean Exposed Liner

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I stopped PB from using acid on liner. But he wants to clean the fiberglass steps with it. Should I stop him? He has already cleaned the concrete that splattered on the outside edge of the steps, and it (cleaning agent) left white spots on the concrete deck where the cleaner(acid?) dripped. He does not want to wash the concrete deck with muratic acid, (says it will ruin concrete) but wants to use acid to clean the steps on the inside of the pool. Is something strange here?

Can the concrete be acid washed? If so, how? Would the coping and steps and so forth need to be taped off, so the mixture would not damage them? Thanks again.