I recently discovered Slimy Grimy for getting rid of rust in tanks. It works great and won’t hurt the paint.

I LEFT THE BOAT in the water for a week of scalloping & Fishing and got the nice stain line on the boat. My neighbor had some Slimy Grimy in a spray bottle & sprayed it on the line along with a rust stain from the dive ladder screws and it was amazing how Slimy Grimy cleaned the boat.