Metal finishing Q&As since 1989

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Baked/Washed metal frames won't take a powder coat finish. Need help!

I have metal tubes that are bent to dimensions. There are left and right side tubes. These tubes will be joined in an assembly process to create a frame for a motored devise.

The tubes were painted in China and I put them through a bake & wash stripping process to get the paint off due to defects. Once the stripped frames were powder coated (by a local professional), I put them into the assembly process and I am finding that the paint is chipping off (flaking off).

What process should I perform to prepare the frames so that they accept the powder coat without flakes? I thought after stripping them and wiping them down with an alcohol pad they would be ready for the powder coating process?

Other processes that I've performed: I've taken the baked and washed frames and I've wiped down with alcohol. I've also sand blasted the frames and they still won't accept the powder coat.

Baked and wash process: To explain my bake and wash process, I took the painted frames that I wanted stripped and sent them to a paint stripper. The paint stripper placed all frames in a basket and put the frames in an oven where the temperature went o 840 degrees F. they stayed in the oven for ~8 hours. The parts were taken out, cooled down, and the pressure washed to get the paint off. Then they were dried overnight.

I need help. Any suggestions?

Sidney Jenkins
3rd party service provider - Duncan, South carolina, USA
April 21, 2010

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April 23, 2010

Sounds as though you have carried out the cleaning process to most applicators requirements. Try this:
1) With a silver coin gouge a groove in the powder coat film.
1a) If the edges of the groove are smooth... the powder is fully cured.
1b) If the edges show a jagged edge then the powder is under-cured and this is the reason the powder is flaking off.
2) If OK and no bad edges then try a coat of wash primer to improve adhesion.
or
3) If your coater has iron phosphate pretreatment facilities try that... zinc phosphate would be better as it also improves the overall corrosion resistance of the coating system.

Terry Hickling
Birmingham, United Kingdom

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April 26, 2010 You didn't say which metal, so I assume its mild steel? We've used this oven process for paint removal before and find that pressure washing isn't enough to remove the residue of paint left behind. We have had lightly shotblast afterwards to get the steel clean, and after that a conversion coating. (could be iron phosphate or zinc phosphate.) Geoff Crowley Crithwood Ltd. Westfield, Scotland, UK It doesn't seem to me that it would be any harder than telling your local powder coater that you want a paint job that doesn't chip off and let him define and execute the process. If he can't do it, find one that can. Sheldon Taylor supply chain electronics Wake Forest, North Carolina April 26, 2010