A properly performed no drain acid wash works very well on dirt stains, recently deposited calcium scale, and iron stains. Recent copper staining might be removed, but older copper stains are more difficult to remove. The younger the plaster surface, the more easily stains are removed. Older pools are problematic in trying to remove the stains.
An important issue is how much acid is being added. Too much acid added, then too much etching of the plaster surface which simply ages and makes the surface rougher and more prone to further staining. Calculate the amount of acid to reduce the TA to zero, and don't add any more than that.
On recently new plastered pools (less than one year) that have "gray discolorations" (and not stains such as from dirt, calcium, or iron), then no drain acid washes will not work. Gray mottling discoloration (due to improper troweling and materials) goes deep into the plaster surface and generally will not be removed, and only etches the surface in trying to remove them. That is not good for the pool.
