UV Radiation Causing Tarnishing?

Somewhere in my memory, I have the thought of UV playing a part in tarnishing during exterior exposure. I was wondering how long these products have been manufactured and has this tarnishing been a problem from the beginning or just relatively recently? Has anything changed in the manufacturing process? It has been a while, but do you think that UV is playing a part in his problem?

Q. In the January, 2007, Painting Clinic, I read the question from C. S., “Clearcoats For Brass”, regarding the tarnishing of the brass parts he manufactures (www.pfonline.com/articles/clinics/0107cl_paint3.html). Somewhere in my memory, I have the thought of UV playing a part in tarnishing during exterior exposure. I remember having to add UV inhibitors to some formulations. I do remember that UV will break down epoxy coatings. I was wondering how long these products have been manufactured and has this tarnishing been a problem from the beginning or just relatively recently? Has anything changed in the manufacturing process? It has been a while, but do you think that UV is playing a part in his problem?

With regard to the “Dry That Primer” question by J. F., there is no mention if the parts are air dried or baked. If air dried, the primer, as you point out, must be cured before applying the top coat. If baked, the primer and top coat, if compatible, can probably be cured in a monobake cycle. What do you think? S. B.

A. I would not expect UV radiation to contribute to tarnishing of brass parts during outdoor exposure. On the other hand, it does contribute to degradation of organic coating films where loss of gloss is accompanied by yellowing, as you pointed out. This is especially noticeable in clear films. In pigmented films, gloss loss is the most noticeable effect. Epoxies are the worst offenders. You are correct, UV inhibitors really help.

I believe the paints J. F. uses are air drying, since there was no mention of an oven. Wet on wet application of conventional coatings is a common practice followed by mono baking, an energy saving technique. But as you pointed out, the coating must be compatible. One of the better combinations is an e-coat primer and waterborne topcoat. In this case, the compatibility results from the primer being impervious to water. That is, it is essentially dry and not effected by the water and co-solvents in the topcoat.

Thank you for your concern, S. B. Also, thanks for writing to Painting Clinic. Other viewpoints are always welcome