Questions and Answers for Painting Anodized Aluminum

 Q. I’m the supervisor of the paint line in a factory in Europe that is applying paints to parts made of anodized aluminum. Suddenly, we are having a problem with paint adhesion. We didn’t seem to have this problem before. However, we did change the layout of our factory production line recently.

Because of the changes in our factory manufacturing sequence and production line, we are storing these parts for at least 13 hours before they are painted. In a management meeting, we discussed whether the storage of these parts before painting, or the anodizing is to blame for the paint adhesion problem. My personal feeling is that the problem is caused by contamination during storage. What do you think? 

A. The short answer is: You are correct. Your problem is probably caused by contamination during storage. Anodized surfaces are some of the best for paint adhesion. In my opinion, your surfaces have become contaminated during the 13 hours storage between pretreatment and painting. The possible sources of the contamination could be many, such a finger grease, airborne contaminating agents (silicones, oily vapors dust particles from grinding operations), etc. It’s well known that airborne contaminants can travel great distances in plant air. Identify the contaminant and eliminate it if you don’t paint immediately after anodizing pretreatment. Otherwise, reconfigure your production line and paint immediately after anodizing. The production line reconfiguration may have moved contamination sources closer to the paint spray booth. Even if you don’t store parts before painting, airborne contaminants can be drawn in the paint spray booth along with the make-up air. However, since you changed or added factory operations, the problem is probably related to that. Otherwise, this problem would have occurred before. 

Pros and Cons

Q. We provide powder coating to a company that does stadium seating, scoreboards, gymnasium equipment, etc. They have a job where a top rail of a stadium will be of anodized aluminum (not sure of the specific process used to anodize) and they wish to powder coat in an orange color. Can you tell me the pros and cons of painting anodized aluminum? Do you know of companies that powder coat such parts? 

A. Anodized aluminum can be effectively powder coated. There are different aluminum alloys, and anodizing can be done with different acids. Adhesion over some anodized parts can be a problem, so you should confirm that this is not going to be a problem for your parts. Be sure that the surface is clean and free of dust, dirt or oils.

Run a sample part and perform a wet adhesion test. Cut the coating in a cross hatch pattern in accordance with ASTM 3359. Put the cut panel in boiling de-mineralized water for 20 min. Remove, dry, cool and do the normal tape pull test.

If you pass this test, it is a very good sign that your part is clean and ready for the powder.