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topic 3184

Primer not adhering to Hard Coat Anodizing


We receive a part from our supplier that is hard coat anodized ( Mil-A-8625 [link is to free spec at Defense Logistics Agency,], Type III). After shipping and some storage time, 1-4 weeks, we paint these parts on our conveyor line. We have a 5 stage spray wash system.

Stage 1 - 130 degree F wash, 9.4-10.2 pH.
Stage 2 - Over flowing city water ambient rinse TDS about 400 ppm.
Stage 3 - 120 degrees F, pH 4.5-5.5 wash with a slight phosphate.
Stage 4 - same as 2.
Stage 5 - Non Chrome Seal Rinse 100 degree F.

The part passes through a dry off oven (conventional) at 200 degree F. It is then primed with a high solids 2 component primer and passes through an Infra red oven. The exit temperature of the part is between 200 and 300 degree F. It is at this point that we see a problem. The primer that is normally a 'little' glossy is absolutely flat, actually chalky. There are tiny eruptions that could be indicating outgassing. The primer has no adhesion or cohesion. It does not ribbon or flake during a knife test, but actually turns powder. It is also removable with acetone in1-3 wipes. If anyone knows what is going on, that would be great. I also have some questions. Could my alkaline clean be collecting in the anodized crystals, not removed during the following stages, and then reacting with the primer? Am I damaging my coating with either the alkalinity or the temperature in my wash line, or the temperatures in my ovens? What temperatures should I stay below?

Any help would be much appreciated.

Kelly Draper
West Plains, Mo.


Looked at your chat on 7-16-99. I am not an expert on paint or anodizing, but have done both. My feeling is that the hard coat should not be cleaned unless it is necessary. Your phosphate prep line is fine for bare aluminum as it will give it a very light phosphate coating which in truth is little more than a very precision very fine etch that gives the "roughness" to the surface for a better mechanical adhesion. Phosphate on iron or steel is a definite very useful prep coating.

I think (magic word) that your slight alkalinity is degrading the very top surface of the hard coat (HC). The acid and the phosphate very slightly degrade it also.

HC is porous, but not to the naked eye and no where as porous as regular anodizing. Since it can be dyed, Poorly, it is very probably keeping small amount of each of your wet tanks and some of the resulting salts that are formed.

I think that this "glop", albeit thin, is reacting with your primer as well as putting a microscopic coating of powder that is sucking up the primer, thinning its bond to itself and the HC in microscopic specks allowing the final paint to slough off. Your 200F prepaint bake will not drive all of the liquid that is in the pores and since it is not sealed , the 300 F final bake is vaporizing that moisture and poking holes in you degraded primer. End result is a bad paint job.

I think that no prep would be better. If you use a solvent, I would try to get one that is compatible with your primer solvent. I think that this is a long version of what Chris Jurey was trying to tell you in a couple of sentences. PS, Chris is a HC expert. He works in a very good shop.

James Watts
- Navarre, Florida


James, Chris, Ted and everybody else that has helped me on this,

The problem is definitely my cleaning line! After trying different cleaning methods, and no cleaning, I have found that both an Acetone [linked by editor to product info at Amazon] wipe, and no cleaning produce good results. The acetone is compatible with my primer so that is the method that I am going to use. (I can not quite sell NO cleaning to my customer.) As verification, and going back to my Chemistry background, I took some primer in a beaker and added a slight amount of the cleaning fluid. BOY did I get a reaction!

Thanks Again! Kelly

Kelly Draper
- West Plains. MO

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