Zero Rust has been mentioned, but I'd like to clarify a couple of things:

While some DIYer's have used ZR as a primer on their project's body, you really are better off using an "automotive paint system", including your topcoat manufacturer's recommended primer, etc., as virtually ALL the compatibility issues have been worked out.

However, for the under carriage, inner fender panels, floor pans, door posts, firewalls, axles, and all those other "working" surfaces, Zero Rust is an effective, easy to use (aerosols or bulk), topcoatable alternative to epoxy primers and other rust inhibiting products. Less costly, easy repairability (if necessary), easy cleanup, unlimited shelf life, AND no Isocyanates or lead, DIYer's are finding it to their liking more and more.

As to "flash rust", which Rob mentioned, our Prep-Step is an excellent way to not only clean-up & degrease steel, but it also phosphatizes and adjusts the pH to ~6, both of which materially enhance paint adhesion (any paint). A major benefit is that it also forestalls flash rust development, buying you some additional time to complete prepping a project before you need to shoot a primer or, as we would advocate, ZR.

Check us out.

WK Irish