Are you sure it isn't salt from the roads? This occurs on alloy rims especially around the spoke holes. Does it clean off OK? If so then a wipe afterwards with an oily rag or a bit of wax polish should help - but keep it away from the brake surfaces!!
have the rims got a clear coat on them? Could be that the salt has got under the porus coating and started to bubble. Sort of like old varnish on a bit of wood.. Can it be picked at with your fingernail or another impliment? I.e does it lift and crack in places?
For Sale Has anyone ever lusted after a Chrome Chopper???