Best thing is to get rid of the rust. Sand blast it if you can. If you cannot afford that or dont want to pull the body to do the underside then clean whatever scale and rust you can, pre clean it and POR15 it. POR15 like mentioned needs rust to "bite" to the metal and it will seal it off. Some people dont believe in POR but I do and have tested it myself so that is the only reason why I do now (I had doubts about it before). If you want to paint over it I recommend using POR15's primer and then your own base coat if you want. Some primers (not sure if it is just me) dont bite to the POR15 and did not work for me. I called and asked them and they said it needs their own primer and whatever basecoat you want to use.