pool stain recurring
This stain (crud growth) is on a modified plaster (many brand names) and I've drained the pool several times in the last 7 years and sanded the spots with a diamond disk. They initially are bumpy and can be knocked off with a sharp chisel, but a drain and sanding leaves the area beautiful and spotless and smooth. After a season the dark bumps begin growing again in the very same exact spots. Could this be a defect in the plaster since new that precipitates a growth? Is it some sort of mysterious fungus or algae buried in the plaster? The rest of the pool is pristine. I have auto HCl and Cl feeders that keep everything balanced perfectly.