Surface rust prep before sound deadning

If there is no oxygen, steel won't rust. POR seals to air.

So does any regular paint. The problem with regular paint is it needs a good priming system to last, so it gets expensive. If prepped right and a good rust proofing DTM is used, such as Rustoleum, or any of its competitors, the metal will be sealed and no priming is required since it is a DTM product.