Internal fuel tank rust can be a real trap, as it often conceals pitting that has nearly gone though the metal. So when the rust is removed, you get pinhole leaks.
There are commercial tank prep/lining kits around, mainly intended for motorcycle tank repair. They tend to be a bit exy to use on a mower though - around the $75 mark or so. POR15, Kreem and KBS are a few of the kit brands.
The prep steps for these are 1. Wash out thoroughly with degreaser 2. Wash out with detergent and hot water, and rinse out thoroughly with water. 3. Etch/rust removal with acid solution, rinse out thoroughly once more.
Just looked at the MSDS for the acid etch solution for a couple of them, and they are essentially diluted Phosphoric Acid, sometimes with a small amount of Nitric Acid. Many of the commercial rust removers are Phosphoric Acid based.
