With gray primer under the black, I doubt it's original paint. I've never seen anything but a thin coat of black on original parts under the hood and interior.
I'm with Gary and use crushed glass at LOW pressure ~ 40# and original paint disappears easily and the glass doesn't remove metal. It will leave a slight texture for the epoxy to bite into.
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Mitch
1970 Chevelle SS
1966 Chevelle SS
1967 Camaro ss/rs
1938 Business coupe, street rod
2000 FXSTS, original owner, 13k miles
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