Easy way to remove the old ashphalt based undercoat is to spray it liberally with WD40 and let it soak for a week. Then it will damn near fall off with a putty knife scraping it. I put some plastic sheeting underneath when i began scraping after a week of soaking with WD40. Every day i would go out to the shop and coat it and walk away. Did that for a week. Then scraped it right off. Lots of it fell off in chunks. Stubborn deposits left after that come off really easy when you warm it up with a bernsomatic torch. Then use rags dipped in laquer thinner to wipe everything down and remove the soaked in WD40 and little undercoating bits that are left.