THIS wORKS !!! and i swear i'm not affiliated with the Company!
I had the same problem as squirrel49. Rough plaster, Calcium buildup with dirt encased and i also have a salt/generator. At first i didn't try much just super shocking without success. Well i followed this thread in hopes of finding a solution. After squirrel49 stated he was going to buy a sander i began looking into alternatives. i came up with H100 acid, but i didn't have success rate to back it up. Then i Came across a Chemical plant in PA that stocked a solution designed to remove scales and minerals from tanks and pipes. After talking to the Chemist he suggested i stick with a pool product and suggested i take a look at a company in FL that makes a great product for removing scales. i visited their website and their it was:"(STAIN SOLUTION #2) is designed to safely and effectively remove copper stains and scale from all types of pool surfaces without draining the pool. In most cases, The Copper & Scale Stuff is more effective than acid washing the pool, and will save both time and money. Please Note: must bypass heater when using this product." i contacted the company and went through an intensive phone list trying to find the product in my area without success. On the second phone call, i requested contacts for the neighboring states. Finally i found a distributor in Boston who gave me a contact in My State (Connecticut) i took the 1hour drive and purchased the recommended dose for my pool along with 2 quarts of purple stuff. A bit Skeptical i put off the project for about a week or so until the wife began nagging me. After lowering the PH, i add the products. WOW! a bit of Hope, while adding The Scale and Copper Stuff i noticed as the powder makes contact with the plaster it instantly turned to Brite white removing the brown calcium buildup until it completely disolved. I continue to brush every morning and run the filter pump as normal. i'm on day 5 now and every morning my plaster looks whiter. I intend to continue treatment for the next 9 days to completely remove all scales.So Far, this has been the best $250 i've spent all year.