Had this issue on a landscape trailer one and found that hitting them with some spray can clear coat after re-painting (or while the wheel is still new) helps keeping the rust from forming.
*Currently trailer-less but shopping
07' GMC Envoy w/ P2 controller (current TV)
88' Chevy K5 Blazer (soon-to-be TV)
74' VW Superbeetle (1303)