I inherited a pool service when I moved in this house. After a couple months, I bought a 3 in 1 test kit. pH was maxed. I wasn't sure if it was plaster or painted, the pool store sold me dry acid. I put the whole 10 pounds in (not all at once) and pH didn't budge. By then I determined it was unpainted, so I bought Muriatic Acid. It took a whole gallon to finally nudge the pH into the center of the test block.