Fresh Air Vent Rust
I've painted over rusted iron (iron gates) with the brush on undercoat. The gate has been outside in a very harsh salt air, high humidity environment for 15 years. The rust has never come through or propagated at all. I can personally vouch for this stuff. It will not ever let rust spread because it contains some kind of rubber compound that seals the surface. Without air/moisture it's impossible for rust to grow. I've used this stuff on everything that is made of metal and it has never failed me. It's a little expensive ($70/can) but that one can can undercoat 20 cars (brush on)!! I do always knock all the loose rust flakes off the metal but in reality any roughness only give the undercoat more to bond to. In fact I never remove all the rust because then all you have is a clean smooth metal surface with no rough finish for the undercoat to adhere to....it will still stick pretty darn good but I like for it to bond with all its' strength.
I've painted over rusted iron (iron gates) with the brush on undercoat. The gate has been outside in a very harsh salt air, high humidity environment for 15 years. The rust has never come through or propagated at all. I can personally vouch for this stuff. It will not ever let rust spread because it contains some kind of rubber compound that seals the surface. Without air/moisture it's impossible for rust to grow. I've used this stuff on everything that is made of metal and it has never failed me. It's a little expensive ($70/can) but that one can can undercoat 20 cars (brush on)!! I do always knock all the loose rust flakes off the metal but in reality any roughness only give the undercoat more to bond to. In fact I never remove all the rust because then all you have is a clean smooth metal surface with no rough finish for the undercoat to adhere to....it will still stick pretty darn good but I like for it to bond with all its' strength.