Letter 11012

Paint adhesion problem 

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I am a manufacturing Engineer at Spirent Systems in Wichita KS. We have been experiencing difficulties painting Alodine coated aluminum. We have been using a Zinc-Chromate primer that does not want to stick good to the surface. We use the tape test and it always fails.

Is there any other primer that might work better? Would it adhere better at a higher temperature, like 80 to 90 Degrees F? We are using Dupont 491-17 Etch-Primer.

Dale O
- Wichita, Kansas

Hello Dale,

The alodine coating should provide a decent surface for paint to adhere to without using a primer. Alodining is a chromate conversion coating, supposed to pass 150 hour salt spray. I'd say quit with the zinc-chromate and try to paint right over the alodined aluminum. I'd also stay away from etching primers, the "etch" is usually an acid, which might negate the usefulness of the alodine. If you can't get good adhesion, try an epoxy primer, providing you are topcoating with an opaque color (epoxies chalk outdoors if a clear or tinted topcoat is used).

Good luck,

Jake Koch G. J. Nikolas & Co., Inc. Bellwood, Illinois

I do not think that your primer is the problem. The first two things I would investigate would be the Alodine film. Perform the test on the bare Alodine and see if any of the coating is removed. If so, then the coating may be too thick and powdery causing a failure at this interface. The second place I would investigate is the part handling. Are any soils accumulating on the surface? Is the Alodine coating given ample time to dry? When the Alodine is dried is it at an elevated temperature? Normally temps in excess of 150 F begin to degredate the coating. I hope this gives you some direction in your investigation.

Best of Luck.

Ira Donovan, M.S.F. Ed. note: Master Surface Finisher is NASF's highest certification level.  Burns & McDonnell    Kansas City, Missouri

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