It says I can use any paint or primer over the Mastercoat Permanent Rust Sealer. Is this really true?

Yes! Mastercoat Permanent Rust Sealer is solvent proof. Once it dries, you can use ANY kind of primer or ANY kind of paint over it. The sooner the other paints and primers go over Mastercoat Permanent Rust Sealer, the better, especially in hot and humid climates.