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topic 18819

Powder coating adhesion problem


I am having trouble maintaining adhesion on my powder coated parts that are exposed to the elements. I am using 360 Aluminum and having them cast using a non-silicone mold release agent as well as having them tumbled in a water based solution, yet when the parts are painted using a TGIC paint I continue to have adhesion problems. What should my pretreatment be?

Thank you,

Robert L Gladfelter
- Littlestown, Pennsylvania


If the 360 is indeed a "sand casting" then entrapped moisture could be the problem. During the curing of the powder the trapped water steams through the powder paint.

Most of the time we would recommend a very light chromate conversion coat but since we already suspect trapped moisture perhaps direct painting over the porous sand casting would give the best adhesion.

In the old days we would say to vapor degrease but EPA has just about taken all that away from us.

So, whatever cleaning and post treating you do, heat it to outgas the pores before spraying on the powder. And be aware that you cannot bake chromate conversion over 140 F without destroying it.

If the 360 is an investment casting, disregard all of the above and apply a very light chromate conversion coating.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide



If the mold release is MoS2 based, problems could also occur degreasing it. I would suggest sanding the part prior to pretreating. Anodising is also a robust system with regard to adhesion and anti-corrosion but could yield problems on cast Aluminium. Anyhow, test another paint (liquid) to check whether it is really a pretreatment problem.

Good luck,

Peter Piessens
- Belgium

Industrial Painting and Powdercoating: Principles and Practices
from Abe Books




I'm a student working on coating of metal substrates. Following letter 18819, I'm wondering what causes the adhesion problem between substrate and coating - whether the steam creates a physical barrier for the coating to adhere to substrate or whether it is due to chemical reaction (e.g. TGIC deactivates due to reaction with steam). Is there any reference/papers I can go to with this?

Thank you very much.

Cheryl Halim
UNSW - Sydney, NSW, Australia

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