Our gunite/white plaster pool is less than 3 years old salt water pool. We have brown to grey staining. Had the pool acid washed then bleached. Mottling of plaster gone but the staining I described is still there. 2 different plaster companies have looked at it.....they have no idea what the staining is. FYI we have copper scuppers and a Three Rivers stone retaining wall. Any thoughts? Has anyone had plaster staining from Bobe copper scuppers? Thanks for any advice/info.