Repair of Powder Surface Defects

Q: We do our best, but we do get some rejects for light coating or blemishes in the finish. I need to know how to rework scratches or other surface imperfections in a powder coated finish and how to effectively apply a second coat when we get a light coat. We would like to have a clear procedure to follow for our spray operators.

A: Repair of a defect usually requires sanding to remove the blemish before application of a second coat. Be sure to use a fine grit for the final sanding and do not leave visible sanding lines. The list below shows a brief description of the repair process.

• All loose material must be removed by wire brush, sanding or similar method
• All rough areas should be smoothed with sandpaper or a grinder
• All dust and dirt must be blown off or washed off
• The part must be dry and free of all debris prior to coating

When you apply the second coat from a defect of a light coat, you will need to recoat the entire part with the powder spray so that none of the original coating shows through. If the second coat is not thorough, you may see areas of inconsistent gloss or a dry-spray look.

Be sure to test the powder to ensure that you can get good inner-coat adhesion between the original coating layer and the second layer. If not, you will need to scuff sand the entire part.

For optimum coverage, adjust the gun settings to reduce the amperage to around 20 to 40 micro-amps and increase the flow rate of the powder by about 10%. Hold the gun a little farther away from the part, about 1 to 2" farther than normal. Finally, cure the part for the normal cure cycle. This process should work well. Practice on some scrap parts to perfect your technique.

It is also possible to touch-up minor blemishes with spray aerosol cans that are color-matched or with touch-up pens. The appearance may be acceptable, but be aware that the performance may be compromised if the coating is exposed to outdoors or a substantially corrosive material.