I tried a similar technique when I began doing my resto a few years back. The Rustoleum seems to have stuck pretty well for the most part. We wire wheeled then sanded with 80 grit before the painting. It doesn't get the rust out of any pits though and it won't stick for doodoo to anything like road tar residue that may have accumulated.
For the price per qt of SPI epoxy primer I don't mess with anything else now. That said IT didn't stick to the road tar residue any better than the rustoleum did. They both blew off in sheets with air pressure alone. The lesson I learned was there is no substitute for blasting that crap into smithereens.

Then I'd brush on a couple coats of straight Black SPI epoxy and forget about it. Gloss finish and all.

Give me ambiguity
or give me something else