Rework and Recoating Powder

Q: We work hard to maintain a high percentage of good parts on the first pass, but we still have some parts with light coating or other minor defects that need to be repaired and/or recoated. We are not always successful at reworking these parts and now we have a second coat which can cause problems with orange peel and other issues. We need to know how to rework scratches or other surface imperfections in a powder coated finish and then successfully apply a second coat. Can you describe the process so that I can pass along a process for my operators for a reliable rework procedure?

A: There are several variables to this (the specific powder, nature of the defect, coating thickness, etc.), so I will provide a simple process that will cover the vast majority of rework.    

• Remove all loose material by wire brush, sanding or similar mechanical abrasion
• Get a smooth surface in the rough areas using sandpaper or a grinder
• Blow off or wash off all remaining dust or particles
• Make sure the part is dry and free of all debris prior to coating

You will need to recoat the entire part with the powder spray so that none of the original coating shows through. If you do not get a complete and even second coat, you will have a dry spray look and uneven gloss. For optimum coverage, there are a few minor adjustments you should make to the guns. This is fairly simple with a manual gun. If you use automatic guns, it is best to batch up the rework and set a recipe for the adjustments.

• Reduce the amperage to around 20 to 40 micro-amps
• Increase the flow rate of the powder by about 10%
• Increase the gun-to-target distance about 1" to 2" farther than normal
• Maintain a constant gun-to-target distance
• Recoat the entire “A” surface

Cure the part for the normal cure cycle. Get your operators together for a brief training session on the proper technique to make sure they understand and can execute it. Practice on some scrap parts to perfect the technique.

One other thing that is good to know is that some powder materials are not great for inner-coat adhesion. They may need to be lightly sanded to achieve a good bond. You may need to do some testing to ensure the second coat will adhere.