I don't know what Hartzell use in the factory but whatever that was, 99% was still there after 6 years. At the overhaul it was just an aerosol can job (with primer first) but that is holding fine.
2-pack paints should be excellent. One can get 2-pack clear paint too.
I'd ask metal prop overhaulers what they use, because they cannot have it all coming off in rain, otherwise people would moan.
Paints are bit of a problem because e.g. the Lycoming grey would come right off within 6 months, and that was on the engine! Lycoming blamed it on a bad batch of paint...