Try to get rid of all the existing rust FIRST ! All you did with covering it with paint is accelerated it. wire brush, side grinder, channel locks or vice grips t get all the loose flaky stuff and then a good pressure wash to get rid of the loose mud and metal/rust etc and open up drain channels. Take it for a good drive to dry all that stuff out, then, put tar or pitch in the holes and rub it in good. That will protect it to a certain extent if you are looking for a cheap easy fix. Go to the dump and get the metal wrap off of all the hot water heaters you can find. Cut/shape these to rivet into 'hiding position' with a good tar/pitch coating on the back side of it. After it suits your eye, THEN spray it with rustoleum neatly. Use a straight piece of cardboard and tape to make straight lines and keep it looking good. Fixed a lot of rust bucket vehicles, floorboards, fenders with this way. The FRAME is an entirely different matter. You need to ensure the load bearing structure of this is sound. Get rid of the rust down to the bare metal and weld in/replace,you can find bed frames to weld in to strengthen channels or same thickness plates to cover holes, extending the new metal well into sound structure, as needed, keeping it aligned properly. Shouldn't use a bubble gum approach with frames, it's not only your life but others you are potentially risking. You would be surprised how easy this is if you know what you are doing. After it's structurally sound, coat it with tar/pitch. It's cheap and easy to effectively put on and it will last.
