If the frame is not rusted through, and is still in good shape, just rusted, then what you want to do is remove the rust as much as you can. I used wire brush in all the places I could reach, using a small one (like a tooth brush also).Then I pressure washed it. Once that was done, I used some rust preventer I got from Oriellys. This stuff is a clear spray that converts the rust to a black base coat, Once that was done and dry, I then used some old time tar base undercoating. I also bought this from Oriellys. After working on an oil rig in the Gulf of Mexico for 33 years, I know about rust. You have to remove it as much as you can, then treat what is left, then coat the steel .