What is that black DTM primer used on late 60's and 70's cars?

[Lizer]

I've encountered this on both my 67 and the 78 LTDII I'm currently working on. Paint and an orange layer of primer beneath that strips off fine with 80 grit, but there's a really slick black primer on the metal that is a real SOB to get off? What is this, and what is the best way to remove it? It's almost like it just polishes it when I sand it with 80 grit.

 

[Barry]

The older cars used were different forms of alkyd enamels.

 

[texasking]

I've encountered this on both my 67 and the 78 LTDII I'm currently working on. Paint and an orange layer of primer beneath that strips off fine with 80 grit, but there's a really slick black primer on the metal that is a real SOB to get off? What is this, and what is the best way to remove it? It's almost like it just polishes it when I sand it with 80 grit.

I've run into this same thing on the square body Chevy roofs I've been doing. Tried everything from 36# grinder, 3M stripper wheels, Ospho, chemical stripper, nothing worked except blasting. Like you said it just laughs at 8" 80#. What's bad is there is surface rust under it, so it has to come off.