OK so we are working on a toy and the pits are fairly deep and filled with rust....If you look in the yellow pages for a sandblaster or media blaster he could remove the rest of the paint and the rust and give your metal an even profile. Because it's a toy they wouldn't charge very much, I'd say between $15 and $30 (not knowing the size of the truck I would say $30 is on the high side. With your metal's profile now being even you would need to prime the metal, i still don't know if you have painting equipment so I'm going to assume that you don't and are going to use aerosol cans of primer and paint. Apply several coats of primer, and sand with a fine grit of paper after it has dried...something like a 320 or 400 grit. If you still have pits after it's been primed and sanded, prime it again, let it dry and sand it again, repeat the process until the surface that you want to paint is smooth....once it's smooth, paint away.
Hope this helps.