This Brick Repair Kit is meant to be used on small to moderate sized cracks on the surface of the firebrick. It is not meant for deeper cracks or cracks that go all the way through the brick. If your firebrick is cracked all the way through, or looks structurally unstable, then that brick should be replaced immediately.
NOTE: This brick repair kit is ONLY meant to be used on L&L kilns. If you use it on anything else you do so at your own risk. It will not work on other types of refractories such as are used in woodstoves, etc.