As someone who tried to battle rust, it's like cancer, it comes back fast. If you are sure the rust is only on the underside, the parts that can be fixed with some more solid metal, i'd consider keeping it. If the rust is showing on the bodywork, get rid of it asap.
I tried undercoating, it looked to me it just made things worse, i know it's a bit of redneck solution, but some oil sprayed on the underside, and driving on the dirt roads did far better job for me, unless you have a leaky engine, then you dont have to do anything 🙂