Report MASK INTERFACING GUIDE FOR FILTERS in How to use this forum Posted April 6, 2020 Use medium weight non-woven interfacing for masks. This acts like a safe filter. Please label each mask with a permanent fabric pen and write "WASHABLE FILTER INSIDE". If they aren't labeled then no one will know about your filter. DON'T USE PAPER FILTERS like coffee filter because they are not washable. The masks are sterilized and washed for the life of the use of them. Thanks!