Metal finishing Q&As since 1989

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Bubble Caused By Pin Hole after Clear Coat

2004

Hello to the person who reads this inquiry.

My name is Sashe and I am a production manager in a aluminium alloy wheel manufacturer. We cast, machine and paint the wheels we produce.

We are facing some problems after clear coating our wheels.

The painting process is as follows :
1)Pre-treatment (degreasing, deoxidation and chromating)
2)Powder Coating
3)Base Coat
4)Design Face Polishing (This is a machining process - by diamond/pcd inserts)
5)2nd Pretreatment
6)Clear Coat. The clear coat is applied by using electrostatic spray.

We notice bubbles are formed immediately after clear coat process. This bubbles we believe may be caused by micro pin holes or the machining process. The problem is that the pin holes are not visible after design face polishing.

Actions Taken :
1) Increase machining feed rate to create rougher surface to basically close the micro pin hole. This has improved but not eliminated the problem.
2) reduce electrostatic air pressure to between 3.5~4bar. Still failed.
3) Tried manual spray gun. Still failed.

I hope you could advise on a type of special clear coat that can solve this bubble problem.

Thank you.

Cheers.

P.SASHENDRAN
Aluminium Alloy Wheel mfgr - NILAI, NEGERI SEMBILAN, MALAYSIA

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Sounds like solvent popping; see below.
https://www.pfonline.com/articles/109806.html

Jim Martin
- Spokane, Washington, USA
2004