Powder coating in very tight corners such as around the spoke nipples etc can be a real problem as the Faraday Effect can cause voids in the corners allowing the powder coat to crack and allow moisture in.
I've blagged the short article below from the internet but there is a lot about it if you look.
Not saying it's impossible to get good coverage with powder but ask the questions of your contractor or play safe and wet paint using a rust resistant primer.
By the way, this article came from an American website called powdercoatedtough.com which has an "agony aunt" called Joe Powder. Check it out.
"How do you prevent “Faraday”?
By “Faraday” you undoubtedly are referring to the “Faraday Cage” effect. This is the issue of penetrating a tight inside corner with electrostatically charged powder coating. The electric field produced by the corona charge delivered by the gun creates a dead space of charge within the corner. This dead space repels the deposition of powder. In addition the air volume exiting the gun creates blow-back the further impedes powder deposition. So not only do you get electrostatic repulsion but pneumatic repulsion as well.
So, how do we mitigate this common problem? A few actions will eliminate this issue. First, use a powder spray gun that has a charge limiter device. Most of the recent models have this feature. It essentially detects a build-up of excess charge in the Faraday Cage and automatically reduces the current of the corona charging tip. This diminishes electrostatic repulsion. If you do not have a gun with this feature simply reduce the current of the gun by about 30 percent. For example, if you are spraying at 18 microamps, cut back to 12 microamps. This can usually be accomplished by simply reducing the gun voltage.
Second, use a high volume/low air velocity powder pump system. The major powder gun suppliers have this option which conveys a very high concentration of powder with significantly less air velocity. This helps with Faraday penetration by delivering more powder and less air to the tight corner.
Third, use a slotted gun tip instead of a fan spray tip. This will concentrate the powder stream to the corner.
Fourth, ensure that you have a good connection from your parts to ground. You can check this visually and also with a megohm meter. You should measure less than 1.0 megohms. It is important to use a megohm meter and not a simple volt-ohm meter. Megohm meters actually put out voltage that more precisely ascertains if an acceptable ground is present.
Joe Powder"
