I was in my above ground pool the other day and I notices little bumps almost like pimples all around the pool wall behind the liner. The pool is an Esther Williams pool and is 12-13 years old. I know it could be a sign of wall corrosion, but this liner is less than a year old, so I was able to look the wall over good before putting it in last year and there were no signs of rust or any type of corrosion. It looked like it was in great shape as far as I could tell. Any ideas on what it could be?