A previous owner had coated the firewall of my 1950 3600 with undercoating, presumably to reduce noise and act as an insulator. The problem was that after 20-30 years (I'm guessing) the undercoating had dulled in appearance and looked lighter in color, plus it had accumulated a lot of dirt that was now permanently imbedded in it.
The removal of the stuff was, to say the least, tedious. I found that Easy Off Oven Cleaner (the unfriendly version, not the newer one) worked pretty well at softening it up if allowed to sit for 15-20 minutes after spraying. Then using plastic scrapers I was able to remove most of it. The small bits remaining was sanded off. It took me an entire weekend but it's gone now. The firewall is primered and will be getting a finish coat of black zero rust later.
The insulation/noise reduction material will be going on the interior.
