I was looking into industrial paints yesterday, the kind of suff that gets used on dumper truck trailers and so on. Came across Besa's
Urki-system, which, coincidentally, contains the paint we use at work for painting chairlift and draglift pylons (Urki-Nox). Very tough stuff (you can hit it with hammers), and goes straight onto steel with no primer.
In that system is their recommended paint for bicycle frames, Urki-dur. This is an oven-drying synthetic enamel, and several times harder than any automotive tocoat will ever be. Primer, topcoat, "that's handy, Harry, stick it in the oven".
I don't have an oven. Well, not one that will take a hardtail bike frame for 20 minutes at 150°C, anyway.
I could "borrow" some paint from work, but riding around with a chairlift-coloured bike would be a giveaway. But I found somewhere that sells it preconditioned in aerosol form (google URNOXSPR), so I have a can of RAL 6018 on its way to me right now. We'll see how that goes.