Eliminate Pin Holes on Painted Wheels

A. @Batummalai Rather than hiding the pin hole we have to control it in melting degassing process and ensure the density is achieved after metal treatment.
Hiding the pin holes is very difficult.
If it is vey minor only it can be hidden in paint clear process.

A. Although surface prep is very important, I think most pinhole defects come either from improper painting/powder coating or from "outgassing" from the substrate. You didn't mention whether you are doing wet painting or powder coating, or any details about it. May we assume that the wheels are not vacuum impregnated against outgassing?

When your rejects are in the 50 percent range from an obvious defect in the process, you must (in my opinion) concentrate on fixing the cause of that defect -- not be diverted into how to hide it, salvage half your wheels, or improve your polishing technique.

A. Just from the information that you give, I'd have to guess a pretreatment problem. It sounds like you have a dip line and the cutting oils (and maybe chemicals) aren't being removed (rinsed) as well as with a spray washer -- can I guess that the pinholes appear in the same area of each rim? If you're not real detail oriented, clear coats can be a pain in the tail. I have an answer to question #2, sell the rejects as seconds, or paint them and sell them. If you can make your cost, at least you're not losing money.

P.S.: If you are using any powder coating (not just clear coat), overspray from aerosol cans (spray paint cans) between paint booth and oven will cause pinholeing. Make sure that there isn't anybody even close to these parts as they are going into the oven that is using spray paint.

A. Dear Sirs,

As Ted has already stated we have to presume that the pinholing is due to out-gassing during the final processing of the wheels (baking).

This is a problem that ALL mfgrs. of alloy wheels have to contend with.

The answer to your problem is to preheat the wheel to eliminate the trapped air (80 °C). Before the wheel cools apply your powder -- a tip ... apply the powder to the wheel face which should be pointing down towards the floor -this will eliminate defects due to dust settlement.

A. Are the wheels being made from machined billet aluminum? If they are, there should not be an outgassing problem, outgassing comes from a material that has porosity. I have to disagree with the outgassing theory and say that machined billet aluminum is not a porous material. Based on my experience, outgassing looks different than pinholes, so there could be some confusion in the descriptions that are being used. Outgassing looks like a little moon crater, pinholing looks like somebody might have stuck a sewing pin or needle into the finish. Pinholes are usually caused by a cleaning, airborne or cross contamination problem (like not cleaning the epoxy powder out of the hopper before putting the polyurethane in), or spraying incompatible materials in close proximity of each other.

Q. We are doing a Diamond Cut Wheels (Mirror Finish appearance). This wheel is Washed Pretreated, Chromed in Spray System and Wet Sprayed With Clear Coat 180 °C. The Pin Holes Appear obviously very bad the moment the Wet Paint Clear Coat is Applied. Wet Paint is done by experienced Painters. The Casting method is Gravity Casting and no vacuum process is used. The Nitrogen Gas is used as Degasser.

Thank You,

Yours Sincerely,

A. The degreaser is most likely your entire problem: either it's not taking all the oil off, it's ineffective, or it has contaminants.
Be sure to do the 'water break' test after rinsing twice following the degreasing step to ensure the material is perfectly clean.

A. You didn't mention what metal you're coating. Using Zn Phosphate I assume its steel?
Do you rinse between stages?
Are they dip stages or spray?
What drying time and temp?

We're just concluding some research on pinholeing and find that there are many causes. The grease could easily be one (as mentioned), but ineffective drying could also be.