Remove the paint with a chemical paint remover. Sand blast the rusted areas. I used a spray on remover from the auto parts place then sanded what was left. It does make a mess. The paint that was still hanging on I hit with a wire brush on a side grinder. If you get to heavy on the grinder on a large panel you can also warp the metal. Blasting is a good way to deal with rust. The frame is a good place for a blaster. That is if you remove everything that you don't want to get sand in. It will work it's way into bearings etc. There are great products out there to deal with the frame but one of em is not bed liner.
