Clean as mentioned above, but then take an old screw driver or ice pick type if thing and poke at it, especially in the corners, poke very hard and literally try to stab a hole through the surface, some places you will (If it is rusted badly) then work at those holes and make them as big as possible with your poker. 

You can also take a ball pein hammer and hit the chassis all over, you should hear a good pong pong sound each time, when you hear a dull thud you will have either found an internal support, very internally rusted spot or a spot where there is a lot of mud or gunge inside. Concentrate on this spot until you have established the cause of the thud. If there is sand/mud/gunge inside, drill a large hole, 14mm or so directly beneath the problem area, if there is already a drain hole it is probably blocked.

Once everything is cleaned up, you can carefully asses the damage. You may consider cutting the holes out or cutting out entire sections and replacing them with new good sections. Anything larger than a finger hole or a mass of little holes must be cut out and replaced.

If they are flat sections, cut back until you cut into good steel, obtain new pieces of plain mild steel, treat them as explained on this page and weld them in place (See welding section).

If your rusty bits include some major piece of chassis like a spring hanger or bumper horn or even a whole cross member, then you must make a few careful profiles on stiff cardboard or sheet steel and take careful measurements, including diagonal measurements to ensure that you fit your new replacement section in the exact same place as the original. It may be a good idea to drill a reasonably large hole in the item such that you can get a small paint brush in to paint it once it is all welded back together.

Take note that there are always a few millimeters of the steel that disappear when cutting a section out with a grinding disk or gas torch.