At some point in my car's past, the PO had the engine bay detailed and then shellacked or clear coated. No surprise that now 60% of that has flaked off and the rest has turned a dirty brown color. I degreased the engine. That helped some, but what remains acts like it won't come off with a diamond-tipped drill. Any advice on how to get the rest of this off?

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1991 300D 2.5 "Rocinante"
2002 Golf TDI "Teen Spirit"
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1984 300D
1966 Mustang I6 3sp
1985 Mazda RX-7 GSLSE
1982 Toyota Supra
1977 Datsun 280z
1971 Datsun 240z