Some while back, I described how the lensing effect of wet cloth can increase the risk of severe sunburn. Beads of water in the cloth's mesh will act as little magnifying glasses and focus incoming sunlight into lots of very small but very high intensity dots. Please see: Risk of Sunburn
Yesterday, I heard a lecture at the Bonsai Society of Greater New York (BSGNY), presented by Mauro Stemberger, International Bonsai Master. His website is: Italian Bonsai Dream
In the course of the lecture, he mentioned that with any plant, bonsai or not, one should not in bright sunlight apply any materials to leaves or needles which, with an uneven surface, can act on incoming sunlight as a collection of lenses, focusing that sunlight into a multitude very high intensity dots and causing leaf damage very much like to the skin damage I had earlier described.
If needed, such materials should only be applied in shade.
A second caution offered by Mr. Stemberger is not to apply anything that will clog the leaves' pores and block the exchange of moisture with the surrounding air.