POR15, undercoat, or sprayon bedliner?

This is just a little blurb from a Porsche site about POR-15. Despite what the company claims, I would never paint or apply any product directly over rust. It's not good to apply directly to smooth "fresh" metal, but properly prepped sandblasted metal it should adhere well to. If you're just going to clean up the metal and apply it, it probably won't adhere very well:

"Bud Osbourne: You must eliminate the rust, unless you want to have to deal with it, again, in the (maybe not too distant) future. A lot of what we see advertised and advocated is touted as a "rust encapsulator" / "rust converter". Even POR-15's name comes from Paint Over Rust, which is NEVER a good idea. If you read, and follow POR-15's application instructions, carefully, you will find that you must remove virtually all rust, and treat the metal with an acid solution (to destroy the remaining rust), before you can successfully apply POR-15. And, if you read the instructions and recommendations, carefully; you will note that POR-15 doesn't adhere terribly well to "clean" (smooth), un-rusted steel. Applied to clean steel, which has been sand-swept, or sand-blasted, however, POR-15 is really rugged stuff."

(From http://porsche356registry.org/resou...pair/325-rust-prevention-myths-a-reality.html )

Just a sample. As with any product, read and understand the instructions thoroughly, and never try to "cover" rust or damage.