Thirty years ago, I decided to restore my Ford riding lawnmower. It had light to moderate surface rust all over & under it. I bought some paint from Ace Hardware called RustStop. I followed the directions to a tee. Wire brush the rust, wash it all with high pressure spray, give it some days to dry & spray or brush paint with Ruststop. After twenty four hours paint over with topcoat paint. That paint job lasted me over twelve years until I gave the mower to my brother-in-law.
The reason I am talking about my old mower is I have thought about doing the same thing to the underside of my truck bed & frame. It would for sure look better & would stop or slow down the surface rust from getting any worst until my son could tear it all down & do the job right.
