When soldering around a mirror, it's a good idea to protect the mirror from the flux, because flux is corrosive. Gunther Seal-Kwik protects mirror edges from "black edge", "creeping crud", or whatever you choose to call it. Unlike spray sealant, Seal-Kwik goes on fast and easy with a sponge-dauber top, with no running, spilling or wasteful overspray. Seal-Kwik dries clear, and will not yellow or crack with age. Just a 4-ounce bottle goes a long way towards solving your creeping crud issues.