According to Musti 1, on YouTube, the best undercoat he has found is chain saw oil. Can be applied with regular air paint sprayer, and he says it dries sticky and tacky and eventually the undercarriage is covered with dust, which prevents water from the steel frame. Cost about $5/gal. That will be my spray of choice from now on.
I reapply every year or so. I only spray undercarriage parts that are susceptible to moisture and the elements. It's not like your typical undercoating or moisture barrier, more like a waxy oil that contains lanolin. Of course our environment in Hawaii is very different from yours in PA. Fluid Film has quite a following, most reviews are very positive if you search.
There is an oil spray that that they us in the rust belt, I hear good things, but needs to re applied each year... I wish manufacturers would just paint their shit and stop with the bs dipping and crap that leaves faying surfaces un treated and allows it to be destroyed by road salt. an extra $500 to me as the customer and never deal with a rotted frame, Im in.... also wire brushing only polishes rust, you need to grind, remove the loose flakes, prime and paint with an epoxy por 15, or heavy duty epoxy coating. IF you leave the rust it will only increase metal loss and you will have a hole or nothing left shortly.