on my old boat trailer I put the white painted wheels and dunked them all the time in salt water. I sprayed them with clear coat in a spray can and was extreamly impressed with how they held up over the years. I dont know if i'd spray your alum wheels but a good wax should help keep them looking new.
*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)