Facing is meant to penetrate into the brick 1/2" or so. When it is fired, it makes that part of the brick harder and more reflective. Be sure not to use too much! Too much in the proper consistency will penetrate too deeply, hardening too much of the brick. Hardening takes away some of the heat-retaining ability of the brick so you just want a bit of it on the inside layer. Too thick and it will harden and fall off after a few firings, taking a bit of the firebrick with it.
- It comes concentrated- mix with water until it is about the consistency of milk.
- Keep mixing it as it will settle out quickly.
- Do a test spot to be sure it is not too thick somewhere on the brick- after it dries in a minute or two you should see no sign of it except a faint milky residue in the pores of the brick.
- Paint/ spray the lid with it- nice even and just one coat. You should be able to see through it - it isn't like a paint.