Factory Undercoat
The way I see it, its best to leave it on unless your masochistic, it's flaking off, or you need to do a repair. I just patch and roll, as its a PITA to take off.
If you really feel the need to take it off, the best way is to heat it up with a propane torch and scrape it off. No need for POR-15, or other high dollar BS. I recommend Rustoleum undercoating for patching as it is awesome stuff, it can be sprayed directly to bare metal, it comes in a 16oz spray can and is frugally priced...
When your done, rub some dirt on it to make match the rest of the underbody. :D
Hope that helps.