My pool/spa was drained to replace the coping which was stained because of a poor concrete mix. While the pool is empty I want to remove the very slight scaling from the tile. The scaling is a 1/4" band less than the thickness of paint. The pool school recommends an acid wash. I don't want to stain the pebbletec so what should be the acid/water concentration ratio. What is the best way to apply the wash? Or, should I just use something like ajax or barkeepers friend? The pool will be refilled Tuesday so I need to get it done this weekend.