A very good effective low tech paint is Rustoleum rusty metal primer. Get it in the brush can (not the spray can, that's a different forumulation). Mix it roughly 50/50 with acetone and brush it on. It soaks in very well this way, and works nearly as well as the much more expensive products. Top coat it if you wish.

'97 T-100 SR5
'86 Toyota's, the variety pack (all gone)