Cracked Mortar Joints
There are cracked mortar joints on the outside of my chimney. Can I use polyurethane to fill the cracks?
Yes, you can fill the cracks with polyurethane specifically designed for concrete. However you can do this only if the cracks on your chimney are minor. If the brick is actually falling out of the mortar, you will have to rejoint the mortar. You may also have to replace the chimney liner inside the chimney chute. If it is damaged, it can lead to deterioration of the bricks.