yes there is a big difference between undercoat and rust check, rust check can take more heat before the flash point (before it catches fire). just think of it this way, anything that gets hotter than boiling water is a bad idea to put undercoat on. your exhaust can somtimes reach 200ºc which is well above the flash point for petrolium based undercoat. even if for some reason the undercoat on that person's muffler didn't catch fire, there is no more petrolium in that undercoat, its all been burned away, maybe just in smoke, not fire, but that can't be good for a muffler. rust check, although not petrolium based, is still not ideal for coating a muffler, the best way to maintain an exhaust system is to just clean it often. as for your oil pan, if the rust is not too bad, just lightly brush the rust off with a piece of fine sand paper or steel woll, than undercoat it, i ran into no trouble when undercoating my oil pans on any of my cars.Now is there a big difference between undercoat and rust check stuff? They basically do the same thing right? I've seen a car the other day with what looks like black under coating all over her muffler. Is she crazy or are there other products that you could just coat everything?