SW-
I used
Peel Away 7 to do the underbody of my 68 Firebird; This stuff worked phenominal. It goes on like a mayo paste, then you place their paper over it... wait a day or so depending on material depth (mine was 1/4" thick). bust out a scraper for the big stuff, and a pressure washer for the other areas (pw not needed, just quicker in some areas). My 30+yr coating rolled off super easy (so easy I used a set of plastic scrapers)
It's bio friendly, so that can keep ya out of hot water in some areas. they sell a neutralizer that will keep it from eating your paint if it touches it- no brainer there. Cost me $60-ish a gallon with enough paper to do the wheel wells and trunk area. Well worth it in my book- very very little associated odor. besides- it beats the **** out of having hot undercoating drop down on ya.
Peel Away 7
PS- It can be found at most paint stores like Sherwin Williams, Finishmasters, etc.