Well, that's not entirely true...
Billet wheels are machined from a solid block of aluminum. Cast wheels are molten alloy poured into a
form and cured into a final shape. During the casting process, micro "gas bubbles" get trapped in the
wheel during the pouring/curing.
What can then happen during the powdercoating process is that the gas is heated and passes outwards
through the metal to evaporate. This is called "out-gassing". If that happens while the powder is baking
it can lead to surface irregularities.
To combat this, most PC'rs do an "out-gassing" process on cast parts before applying powder. (They bake
the parts twice w/o any powder on as part of the prep).
I've had quite a few cast parts done without issue over the years. There are also many people that
powdercoat their stock wheels which are all cast.
The only thing I can't vouch for first hand is a cast part with chrome on it. Chrome is a plating layerd
on top of the part. So if out-gassing occurs, I can possibly see a potential where it may mess up the
chrome and subsequently the powder.
This may not be much of an issue though because I know many people and powdercoaters who coat
over chrome all the time, including chrome wheels.
9 out of 10 chrome wheels/parts are cast (if aluminum alloy). Generally most manufacturers do not
chrome over billet because it does not hold very well and eventually always "flakes" off. Billet
wheels/parts are generally "polished" because of this.
