Automotive paint is 2 parts, paint and hardener. thats about the only way to make it gas proof. Or have the tank powder coated.
back in the 40s and 50s, people used to brush on automotive paint. with a good quality brush and a lot of tallent, mixed with a fair amount of buffing, it should work fine.
I like Valspar, the trick to a fuel proof paint job is long cure out . I bought a spare tank, scuff sanded it, sealed the seams and studs with JB Weld, sanded cured epoxy smooth, than applied several coats of Valspar Red Primer. After a 2 week cure out, I sanded the red down to bare metal, to remove high spots, than applied many thin coats of Valspar Gray Primer sanding out after several hours drying in the sun. When it looked good, I let the Gray primer cure for about 3 weeks, than applied 3 thin coats of Valspar Satin Clear, to seal primer. Later the tank was painted [ Valspar ] Gloss Black, and Gloss White, and was put aside for the rest of the summer. Next season, I bolted up the painted tank, and had no fears of spills, though I was very careful not to fill tank more than 2/3 's for the first rides. Yes, it was a pain, but the results were a beautiful tank with a tough finish. I've tried all kinds of paints over the years, and most will prove somewhat fuel proof, if they can dry long enough. But I've yet to see a paint that's fuel proof, without a long cure out.