RE-SPRAYING PARTS

Q. Please answer these questions for me. I was under the impression that if you wanted to rework and re-spray over a powder coated part the following steps should take place for the best results.

1) Sand down the affected area (better yet, sand the entire part to a flat texture, removing the gloss surface of the coating so that the new coat of powder evenly adheres to the previously coated surface)

2) Wipe the sanded part down with a tack cloth or if it is a really large surface; send it through a rinse and dry-off oven (tacking any water spots that may possibly remain after the rinse) prior to recoating

3) Adjust your spray unit, reducing the spray gun voltage (since you cannot spray the coated parts on the same settings that you are using to spray bare metal); this will prevent the paint from being repelled from the part since the first layer insulates.

4) Use the proper oven settings & cure time recommended by the paint manufacturer to cure the repainted part.
Now here is what I am watching and what I am being told...

The line supervisor is sanding the affected area of the part to be reworked only. My concern is that most of the parts have a high gloss coating on them. He is sending it back through our three-stage pretreatment system, which consists of iron phosphate cleaner, a rinse, and omitting the third stage, which is a sealer. He was sending it through all three stages and I finally won the battle to turn off the “sealer” stage. I am still trying to figure out what they are phosphating, but I haven’t won that battle.

Their response for running it through the iron phosphate again was that they were?“re-etching” the part so the paint would adhere to it when the part was re-sprayed.

And another concern is that it may take them as much as re-spraying it a third time to finally get a “passable” part—passable in their minds, not mine. I would question adhesion of THREE layers of paint, since the first two didn’t apparently work.

I have tried to find some source that explains the rework/re-spray process but have not been able to find any such resource. I am only going based on processes I have used in my 12 years of experience in powder coating. And never would I re-spray a part again, if the first time did not work, without just stripping the part and starting over.

I also know that some coatings are not recommended for re-spray due to slip agents in the powder. Of course, they would have to refer to the technical sheets for the powder that they are using, and that is rarely done, either. Usually, the tech sheets are a critical reference when even coating the part for the very first time in order to have the correct formula of heat and curing time to do the part correctly. I am at the end of my rope on approaching this subject. Can you help me? Thank you for your time. G.M.

 

A. Your process for recoating is sound for the most part. Certainly you would sand any area with a blemish and sanding the balance of the part may be helpful, although it is not always necessary with powder products. Many powders will chemically bond to the first coat. With epoxy powder you may have more concern than polyester. The inner-coat adhesion of any powder should be tested before applying a second coat to a cured film. If the test shows good results then the part does not need to be sanded before recoating. Cleaning, rinsing and drying are also a good idea to remove any dust or contamination from handling. Iron phosphate is not useful; it will not etch the powder surface or contribute to adhesion in any way and it can cause water spotting that could cause additional rejects.

The gun settings should be changed for recoat. Many guns have a factory pre-set for recoat that controls the current draw to prevent back-ionization and improve coverage. If the gun does not have a pre-set for recoating, the voltage should be reduced to around 55–60 kV. The gun should be held around 2 inches farther from the target than usual, and I recommend an increase in powder output of around 10%. The part must be covered 100% to avoid surface roughness.

A third coat is rarely successful or desirable. The inner-coat adhesion may be acceptable but it will leave a very heavy film and it will be hard to avoid back-ionization or other surface blemishes. The orange peel will be heavy and the reject percentage for a third coat will always be high. I hope that this information helps you in your quest to improve the process.