Go for it. Clean rust area again, apply an acid etch primer lightly to bare metal. Add a coat of primer/sealer and let flash (or dry to recomended time) then top coat with paint. After paint has cured spray your rubberized undercoat on and your golden.
I usually spray industrial enamel on trailers and flatbeds and truck frames. Its super tough and durable, but it will rock chip after a while.
The undercoat helps to cushion the rock impacts- just be prepared, its expensive, probably close to $20 a can for big cans of good stuff & you will probably need 6+ cans. A newer crew cab truck frame (bare with nothing on it) will take 3 cans that are applied rather lightly.
Best of luck!
MillerTime- Future Sabre Handler!!