I soaked some shavings of that potting in 4 or 5 different solvents I had in the shop to see what might dissolve it (I skipped lacquer thinner because I didn't want to destroy the paint). Nothing was highly effective but plain ole gasoline was the best. I used gas in a squirt bottle and a plastic putty knife. And a LOT of tedious, careful scraping. Took an hour plus, but I got it all off.