Your high builds are typically very porous and thus do poorly at sealing the metal. As suggested by this from the ACME FP415 TDS:
"* For complete refinishing of larger bare metal areas, it is recommended to use an etch primer to ensure complete corrosion protection."
Since etch primer is so 20th century, I would recommend an initial 3 coats of epoxy primer to seal the metal from any moisture intrusion. Some, like SPI and H/K already have higher build characteristics than most epoxies, and can give a more stable substrate with the benefits of using a high build. Of course all of this assumes that you are taking things to bare metal.. If you are just scuffing a factory finish, then the high build should be fine over that substrate.
I always said Eastwood bought a couple sea containers of cheap paint rollers and the marketing geniuses that they are, figured out how to sell them at 1000% markup
But in all seriousness, you should be able to brush, roll, or spray on an epoxy or whatever finish. The hard part is going to be establishing a relationship with a shop to do the topcoat, as most won't trust the prep beneath unless they did the work themselves. Too worried about any primer delamination as any poor prep that occurred prior and because they did the final finish, it's always "XYZ shop painted it for me". Granted, you can roll on a urethane finish and block it out as well.
At the end of the day, no matter how unorthodox the application method may seem to some, I would use the opportunity and select the best product you can afford, given how labor intensive this will be.