I looked through several other threads but couldn't find any discussion on how to remove the thick (anywhere from 1/4 inch to 1/2 inch) rubberized undercoating that has been sprayed inside the fender well, trunk panels and along the floor pan. I tried some chemical strippers but that doesn't even make a dent, I tried sanding it, it gums up the pads after about 20 seconds, I've tried scraping it with sharpened putty knife and have gotten maybe a 1 x 6 inch section down to bare metal after about 2 hours of scraping, I even took a multi-tool with a serrated blade and tried cutting through it and the heat from the blade just melts the stuff all over the blade.... not afraid of hard work, but this crap is like working with hard plastic coating and soft gooey inside...I've got the entire floor pan, fire wall, trunk pan and up inside the rear fender walls and wheel wells on my 50 Shoebox...any suggestions?

Heat then scrape it off with a putty knife

Agree. Got to soften it up with a torch and then scrape it off.

Yep, torch, scraper and..... a decent size fire extinguisher.....

That's probably a rubberized sound deadener, rather than undercoating. Someone
has spent considerable time and money to quieten down the Shoebox Ford. If this
is a driver, why not leave it on, or at least some of it...? It does work well for giving a
quiet ride.

But if you must remove it, what's been stated is the only way (heat and scraping).
Some of the low-end stuff ignites at around 500*F; the high-end is good up to
about 1100*F. Use a propane torch and I see you already have the necessary
sharpened putty knife. After scraping, the residue will clean up using acetone or
lacquer thinner.
We'll think good thoughts for you...

Thanks for the feedback (and good thoughts.. ) The majority of the stuff is in good shape, but there are a few areas where I replaced some sheet metal that had rusted through (some of the pin holes I just sanded and used a lead free solder fill.) i suppose I could just re-apply where needed. My concern is primarily aesthetics...just looking for a uniform in color surface. Is this stuff okay to spray primer/paint on? I'm changing the entire color scheme on the car and while most of it will be covered with carpet, there are some spots in the trunk along the fender walls between the wheel wells that I won't be able to carpet so I was thinking I might just be able to color match with paint...am open to suggestions if anyone has any. Thanks again for helping a noob...