Another method for removing the stuff is a power washer. I spent $5 at a self service car wash...took all the loose stuff right off.

Don't go knocking that undercoating either...the rubbery black stuff they put on underneath does work if applied properly. The key is nice thin coats just like paint.

When some hack artist does it, they spray the stuff everywhere and put it on way too thick....very bad.

When done right the rubbery finish of the stuff protects against small rocks and gravel that get kicked up underneath by absorbing the small impact and preventing the rock from scraping the paint off.

When it's done wrong (and it usually is) little crevases and bubble get formed that actually trap moisture in the worst places.....and in no time at all the body and frame start to rot.

The milky white junk they spray in the door jams is garbage though.