Failure of polyurethane coating on aluminum substrate

A. It could be several things. And more likely it will be a combination.
Aircraft are painted with polyurethane topcoats so I don't really think that it was a ridiculous choice of coating (unless leachates from concrete attack polyurethanes which is outside my area of knowledge). So my first port of call would be the pre-treatment. If the pre-treatment was not of the highest standard then you may be getting cracks through the paint allowing moisture through which would cause the paint to flake off. I would have expected this to be initiated where there was the longest contact with moisture (around the base).

Could damage have been done to the paint during installation? This may have been an initiation point for the moisture to get in. I am assuming there are fixings holding the lamp to the base.

A. Hi Bhavin,

You will need to give us a lot more detail before anyone can give you a sensible solution to your problem.

Firstly, what anodising is it (chromic, sulfuric, hard, sulfuric-tartaric, sulfuric-boric...)?
Secondly, are you anodising yourself or receiving the parts anodised and then painting?
Third, are you using a primer prior to the top-coat?
Fourth, what is the length of time between anodising and painting?
Fifth, are you doing anything to the surface prior to painting (solvent clean, abrasive, surface activation...)?

If you let us have these details I am sure someone will be able to give you a solution to your problem.

A. Hi Manu. Although anodizing may not be an option for you for one reason or another, I question the premise that it reduces the reflectivity too much. Millions upon millions of reflectors have been built from anodized aluminum for outdoor lighting, streetlights, doctors' & dentists' lights, indirect lighting in homes, infrared reflectors, and many other applications.

People are very welcome to suggest generic types of clearcoats that may work, but not brand names

Regards,

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Hi cousin Mayank. A general "steering" clue that we can offer is that 90% of a successful coating process is the pretreatment. Do you already do the required mechanical finishing, and a good chromate (or chromate substitute) pretreatment process? Please explain what you do in terms of mechanical finishing, chemical pretreatment, and inspection before you will apply the PU paint. Will there be a basecoat/primer of epoxy under the PU? Are you intending to spray two-component PU coatings? We assume that you want a thermoset rather than a thermoplastic coating? Best of luck!

It may be that a supplier of the PU coatings can visit you and get you started; if not, you'll probably have to hire an experienced consultant as there are obviously scores of things to consider and early decisions to make. Myself and other readers stand ready to try to help; but while we can try to answer a particular question in a highly specific situation, it isn't possible to do general training and planning via forum responses. If you are looking for answers that contain more words than the question, you probably need a book, training course, or consultant rather than a forum response :-)

Regards,

A. Hi Ajit. Photos would probably help, but in general a polyurethane coating turns white when there is too much water vapor in it, and it becomes sort of an oil-&-water emulsion. The water vapor is probably coming from the aluminum. Do you bake it to drive off any water vapor before coating?

I know that white water rings on polyurethane coated wood furniture can be removed with a hot air gun. Although I don't know whether that would actually work on your aluminum parts, you might try it just to convince yourself that water vapor from the aluminum is in fact the cause of the discoloration.

Regards,

A. Hi Ajit. Thanks for the pictures; this doesn't seem to be an emulsion problem like the white water rings seen on wood furniture.

To my eyes, there looks to be filiform corrosion along the nailed seam in the pictures with the stainless ruler in them. I suggest that you look up some images under that topic to confirm whether you agree or disagree with me. That may give you some hints about possible causes.

But that's probably only one of the problems. Per my response to Mayank, an old truism is that 90% of finishing problems are the result of insufficient pretreatment. What do you do to this aluminum before applying your air-dried lacquer? And is it chromate conversion coated? It might also be helpful to know what alloy this is.

Regards,