It did a fairly good job taking rust out of a Firestone tank I have, but I also used lockwashers and shake/shake/shake several times. I filled the tank and let it sit for 2 d at a time. I started a 2nd gallon after the 1st one was completely black and I didn’t think it was doing anything more. I did put Red Kote sealer in the tank afterwards. It did work great on superficial rust on the outside of the tank with soaking for only 1-2 hr. And yes, the remaining paint and decal weren’t affected at all. I still have my 2nd mix (concentrate, $19.99 at AutoZone) and plan to continue using it. Neal