Most alloy wheels are painted, a finish that consists first of a primer sprayed onto prepared bare metal, followed by an automotive style paint and a protective clearcoat that seals the wheel and finish against water and air that can cause corrosion. Wheels are painted with an HVLP (High Velocity Low Pressure) spraygun, in much the same process by which auto paints are applied. Most original equipment wheels are sprayed with a liquid clearcoat, however many refinishers now use a clear powdercoat which is baked onto the wheel for a finish that is even tougher than the original.
The pictured BMW wheel is finished with a full face paint in a standard BMW flat silver. (Click here for a larger version.) Note that the color is uniform across the entire wheel. This is a "full-face paint" finish, as opposed to a "flange cut", where the outer edge of the wheel is machined. Not so long ago, painted wheels came mostly in shades of silver with an occasional white, black or red wheel. Now there are many new types and colors of paint, giving many more and different effects. Many people like to paint their wheels in different colors - often an anthracite grey, gun metal gray, or even a plain black or bright white. Some of our customers have had their wheels painted the exact same color as their car! It's often surprising what an effect this kind of thing has on the "look" of the car. Using even a slightly different silver, for example, tends to make the car stand out, but in quite a subtle way.
I often see wheels that have been damaged by brushing against a curb or other road hazard, scraping the finish off the outer edge of the wheel and damaging the underlying metal, a condition we call “curb rash.” Other types of damage include scrapes across the spokes and damage from improper use of mounting machines or torque wrenches. Unfortunately, there is virtually no way to touch up such damage. Proper application of paint and clearcoat means that both must go onto the wheel as one coat. To simply touch up a single damaged area will leave a discontinuity between the different applications of clearcoat, which will eventually allow corrosion to enter. As well, aluminum alloy that has been exposed to air begins to corrode almost immediately, leaving a microscopic layer of corrosion on the metal, which will prevent the finish from sticking correctly.
To properly refinish a wheel, the wheel must be bead-blasted back down to the bare metal, and is usually run on a CNC (Computer Numeric Control) lathe to smooth out any damage to the metal. Especially deep scratches can be built up by welding and then lathed down to the proper surface at this time. The wheel must then immediately primed to prevent the corrosion layer from forming. Priming, painting and clearcoating must all take place in a substantially dust-free environment, or the resulting finish will be speckled with dust particles.
All this does mean that refinishing wheels is not particularly cheap. Refinishing wheels properly will generally cost somewhere in the $200 range, although the very high cost of original equipment wheels ($500-$600 new) generally makes refinishing your wheels or buying already refinished wheels very cost-effective.
Any clearcoated wheel should be cleaned with a product that is non-acidic and non-abrasive. Many commercial products sold as wheel cleaners unfortunately do not qualify as either of these. Any product that says to spray on and remove within 2-5 minutes is probably a low-acid solution, which burns off brake dust very quickly, but also eats away at the clearcoat. It doesn’t take very long at all for such cleaners to get under the clearcoat and begin to kill the finish, as well as allowing environmental conditions to corrode the wheel. Acid damage will therefore show up very quickly on painted wheels looking like white spiderwebs beneath the clearcoat. Some full-service carwashes will use acid-based cleaners to clean wheels as quickly as possible. Be careful out there!
The products that I like best for clearcoated wheels are P21S, Simple Green and Wheel Wax. Wheel Wax, designed for application on clean wheels, works to prevent brake dust from sticking to the wheels in the first place, and making particles that do stick easier to remove.