I've been doing this for weekends now..man what a pain, only i didn't have factory undercoat, i had this thick, greasy, sticky tar like garbage someone sprayed in every seam ever made(74 Challenger) and i have wire wheels of different sorta, scrappers, flat head screwdrivers, wire brushes,
What i did was take the parts-cleaner fluid that i was going to dispose, put it in a spray bottle, and sprayed it all over the underside of car on stands.. Most of it soaked in after a few days, got soft and scrapped off, some areas were still tight, theres where the tools come in, I have the leafs/axle out right now.. and doing all that, i swear those truck extentions are a nightmare, the body grooves and pinche welds and tight spaces...ugh..after big work, hit it with brake cleaner ( a few cans) then let it dry..then degreaser... then water..soak it..and wash it off.
wipe what you can, dry it out and hit with whatever, por15, zero rust..etc