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Automotive paint suppliers will have "chassis black" paint. Check them and see. if you're going spray bomb then semi-gloss is the way I'd go.
I had to change the bent frame under an F150 I had bought wrecked. I cleaned/wire brushed/sanded/primed/painted the "new" frame, took all of a week in the evenings. Beautiful ! Show quality, almost. In 6 months it looked like every other F150 on the streets frame. Never again ! !