In the January 2001 Painting Clinic, I answered a question about iridescence in ultra-thin coatings applied to aluminum mirrors. The question posed by J.C. was as follows: "I try to deposit a thin layer of varnish (5-8 microns) onto an aluminum mirror. So far I have had some problems since I see iridescence once the coating is dried, even with a thickness up to 15-20 microns. How can I solve this problem? Thanks for any help."
Although I am not an optical physicist (where is my son in-law when I need him?), I believe the phenomenon results from light reflected at different angles (as from a prism) by some crystallinity in the film. My guess is that, in this case, it is caused by the filler in the varnish. Fillers are often added to varnishes for flow control. I suggest you either change the filler in the varnish or the varnish itself"
On reflection (no pun intended), I realized that only half the answer was given. Although the crystallinity in the film causes the optical interference (iridescence), the real reason is the low coating thickness. According to my rounded-off calculations, if 1 micron equals 0.0394 mil, then 5 micron equals 0.1968 mil, and 8 microns equals 0.3149 mil. At these calculations it is possible that filler agglomerates can be thicker than the resinous part of the coating film. They would stick out of the film causing the apparent crystallinity. To correct the answer, add "increase the film thickness" to the aforementioned suggestions. I hate to be half-right. It's almost as bad as being half-slow or half-____.