anyone have any thoughts on what is good and what is bad?

one time dealer goo or aftermarket annual spray?

The waxy stuff GM has used for years is the best, you will see in on strut towers, under the trunk lid seeping out around the reinforcement bars. It seeps into the seams and stays there, repelling condensation. Eastwood sells it, its self healing, so they say. Never gets real hard and is supposed to flow back into an area where it got scrached off.

Except areas like fender wells that take a beating tar type undercoating is a good thing to stay away from. Eventually moisture gets under that thick film of tar and rust runs rampant. Ive seen the steel of entire wheel houses turned to crust and you would never know it by looking, untill you push against the undercoating with your hand and find there is nothing but huge flakey c-rust behind it. This was what my 90 Regencys front inners were like at 300,000 miles 1.5 years ago when I bought it. All fixed now. The rest of the car was clean as a whistle, just under the heavy "undercoated areas" was gone.

Its never a good idea on a used car to put any undercoating over already formed rust, it will just seal in the rust making a nice little incubator for the little rust varmits. Rust Lives.....................

the only stuff you will ever need again for rust is this paint called POR-15 (Paint Over Rust) it comes in a few different colors, and forms a shield over rust, stopping it in its tracks... no flaking or peeling, and can be sprayed or brushed on......right now, its basically the only thing holding my friends 88 monte carlo together