I have used Kreem with mixed results. It works well but make sure the tank is completely clean, and I mean spotless inside before coating it or the coating will peel off and clog everything. If I remember right the cleaner is muratic acid and it will damage the paint. Mask the paint with tape and paper. I cover the openings completely and cut an opening so acid can't get on anything even in case of a spill that would run down the side or spatter.

I learned all I need to know about life by killing smart people and eating their brains.
Eat right ,Exercise ,Stay fit, Die Anyway!