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Try American powder coating coating in kent ( I think thats there name). they are in kent look in the phone book. When I used to live up there, they did all my antique cars.
Talk to Jeff Flint (forum handle jeffvette - you can email him, but not PM him). He does a lot of powdercoating. He's down in the Vancouver/Battle Ground area, but it might be worth your time to ship them. If you have problems getting hold of him, PM me.
So wouldn't a good powdercoating company understand how temeratures affect aluminum alloys used in wheels and use a curing temp and coating material that would not cause problems?
There are a lot of people using aftermarket wheels for racing activity that are powder coated (AFS for one example) and I have not read anything on the CF about specific wheel failures where the wheels are powdercoated. I have 4 years worth of trackdays and autocross on my AFS wheels and there is no visible indication of cracking.
Actually, if you read that thread, powdercoating is not what caused the problem. It was powdercoated, and there was a problem (it's unclear as to what the problem was, but I suspect the customer wasn't happy about something). So, they heated it and removed the powdercoat to do it again. It is THAT process that is suspected to have ruined the integrity of the wheel.
So the moral of the story is do it right the first time