partagas, I love your avatar! I watched something on Trucks! on SpikeTV where they used some kind of paint on primer/rust-be-gone type deal (looked like painting on tar pretty much) that they said worked better painted over the rust itself (after a quick brushing).
Trust me. I've done an underbody and it's still rust free 15 years later. You can skip the second step but I say why? I could list a few things you don’t want to get these products on but these come to mind first. Your paint, exhaust and drive shaft. Most of these are apply and drive after a few hours but stay out of water and dust.
Wire brush and scrape off as much rust as possible.
Get a spray bottle and buy a product called (Ospho). It's a green watery liquid that when sprayed on, turns iron oxide aka rust, into iron phosphate an inert white powder. If you cant find Ospho? Stay away from the jellies. Get a liquid that turns oxide into phosphate. It's user friendly but try to avoid getting excess amounts on your skin. Follow instructions for dry time.
Next buy (red oxide rust inhibitive primer) Not the white stuff and preferably not in spray cans. The red oxide primer is the same thing you see on structural steel. Brush it on and apply a second coat if you want the peace of mind I do. This stuff dries to touch after a few hours but cures very slowly. Follow the instructions to the fullest and then some to make sure it's fully cured before the next step.
Get the rubber underbody spray and spray until your hearts content. The applications I described will last many years. I plan to do a good underbody cleaning and just the rubber underbody coat in a few weeks. I've no rust but the factory did a half a$$ application.
I'm sure your frame is rusting also and if you want a nice new look follow the steps above except right before you do the rubber coat apply two or three coats of Sherwin Williams industrial acrylic to the frame. I did it on mine and it's mighty tough stuff…….Mark