Not cheap but this is what I use. Clean (no residue solvent) and spray all interior parts. Wait time on instructions, well ventilate area and weld.
http://www.eastwood.com/eastwood-s-self-etching-weld-thru-primer.html
Mike


1951 Chevy 3600 rust bucket with GarWood dump bed conversion
http://www.flickr.com/photos/mikeybs51chevy