Enamel or Undercoating

I'm in the process of cleaning up the underside of my truck. After removing the bed (to replace the sending unit, fuel lines, shocks etc.) I'm going to clean/brush the frame and associated parts. I'm thinking about using rust-o-leum with a primer or primer and undercoating spray. This will all be from spray cans.

Any suggestions? Thanks!

I ended up using primer and enamal on the frame and primer and undercoating on the underside of the bed.