I need to paint an exterior door that not only is metal, but has a small amount of rust on the outer edges. The door is used quite often and takes a lot of wear and tear. The past paint jobs have not held up very long and I have been given the task to prevent this from happening again. I primed, painted and put a laquor finish on the door, thinking this would do the job. Nope, didn't work! Any suggestions?
Priming and Painting Galvanized Metal, condensed from KILZ.com, other manufacturers also make specific paints and primers for galvanized metal.
The galvanizing process, which is designed to prevent rust, leaves an oily film that can prevent coating adhesion. The zinc in galvanized metal can produce a milky “white rust” (which is common when it has weathered) that must be removed with a stiff brush or abrasive pad, prior to coating. It’s necessary to remove the oily film that can prevent coating adhesion with a water-based cleaner/degreaser.
Prime the surface with a water-based primer. Since oil-based primers can interact with the zinc in galvanized metal, causing premature peeling, always use a water-based primer when painting galvanized metal. KILZ® 2 Primer is an excellent choice when priming and painting exterior galvanized metal. It will grip to the metal surface and provide a better surface for the paint to adhere.
Wirebrush the rust to remove it. Apply a primer designed to adhere to bare metal. (I'm fond of those which react chemically with rust to help finish the job of preparing the surface, but those are only available in dark colors as far as I know.) Then apply a paint compatible with that primer -- not all paints layer happily on top of each other; generally you want the topcoat to use the same solvent the undercoat did for best adhesion.
Theoretically, painting a metal door is like painting a metal car... I wonder what an auto body shop would charge if you asked them to paint it.
After sanding all the rust, apply a rust neutralizer, such as loctite rust neutralizer. Spray it on, read instructions. then follow that with either priming the surface or applying some auto body filer, such as Bondo, If the surface was compromised from the rust. Sand the body filler until it is smooth and flush with surrounding metal, then primer, then follow up with finished paint coats.