Q. To preserve the schach mats made of reeds from getting moldy when wet, or the strings and straps that keep them together from rotting, can one spray them with transparent shellac, lacquer or varnish, before and after using them?
A. On question 876 in regards to if there is a problem with using bamboo that is coated with a varnish like substance for schach, we wrote; Poskim permit painting or coloring the material used for kosher schach since it does not change the nature of the substance and it does not create a separate-standing body. (Haelef Lecha Shlomo 365, Beis Yisroel 106, Nitey Gavriel Sukos 15: n. 15 -quoting Chazon Ish, Orchois Rabbenu quoting the Steipler Rov ztíl,) so too is the opinion of Horav Shlomo Miller Shlit'a.
Horav Shlomo Miller Shlit'a added, that even if the spraying is not done for aesthetic and beautifying reasons, but rather for structural maintenance and building preservation, if the resulting spray cover is so thin that it can not be peeled off as a self standing layer, as it crumbles into unsubstantial particles, it is permitted.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a