I restored my first as well, a 66 Tempest, 2.5 years ago. I too had "surface" rust underneath on the frame rails and the underside of all of the floor pans. Nothing egregious, just your typical surface. Since I had no plans to finish the underside with the showroom shine you see on some of the more top dollar restorations I simply coated everything underneath with a few coats of the rubberized rust encapsulater you mention. It pretty much turns the entire underside into a relatively hard black surface with some texture to it. I did some mild brushing with a wire brush prior too as well, just to make sure anything loose or other debris was removed. As I said, that was 2.5 years ago and it looks as good now as it did when I finished it then. Certainly I'm not implying the rust is gone or anything similar, but the stuff does work pretty well as I can attest to. In my case the car is always garaged as well though and not driven when it's wet outside, albiufn it definitely is a driver and I take it out whenever I can for a spin. I'd say go this route if you're looking for relatively cheap and quick way to put a solid coating on everything that protects and looks halfway decent in the end. My two cents anyway.