Going to be working on get my trailer roadworthy for next year and one of the projects is going to be sanding off the surface rust and refinishing it. Seem to remember a product out there that once you have the surface cleaned you painted this stuff on white, and when it cured it had turned black. It was some sort of a rust inhibitor/primer that really sealed things up to keep the rust from coming back.

Anyone know what this stuff is, or have any recommendations on other products that aren't going to require an EPA permit?

Thanks