Q. Are schach mats that have turn partially moldy kosher for schach? They do not smell to mold, but you can see the discoloration. Can you paint them?
A. Horav Shlomo Miller's Shlit'a opinion is that the mold should be removed (including for health reasons), they can then be painted. (See question 1884 above).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a