Can I use Mastercoat Permanent Rust Sealer on a gas tank?

Yes, by all means. Mastercoat Permanent Rust Sealer will seal a gas tank inside and out. Mastercoat Metal Prep & Rust Remover is used first to etch the inside of the tank. Mastercoat Permanent Rust Sealer SLIM is the optimal choice for coating the inside of gas tanks, since it sloshes easier for more even coverage.