I would recommend cleaning out as much scalely rust as possible. The methods mentioned above will work. Hose out the dirt and grime, let dry. Then treat rust with a rust converter like "Must for Rust" (Lowe's) or Rust Oleum's rust reformer then paint. Mostly because if you paint over rust, all you really get is painted rust. The rust doesn't need painted, the actual metal needs to be painted.
2016 Ford F250 XLT 4X4 6.2L
2015 Rockwood Signature Ultralite 8329SS
Nights camping in 2016: 19 nights
Miles traveled in 2016: 2,575