Why brick fails from water, not fire
Water enters porous brick, expands and contracts with temperature swings, and pops the face off the brick. That is spalling, and it is the most common reason a chimney ends up needing a rebuild.
Breathable sealer only
Paint and non-breathable coatings trap moisture inside the brick and accelerate the damage they were meant to prevent. Chimney waterproofing uses a vapor-permeable siloxane formulation designed for masonry.
Repair first, seal second
Sealing over open mortar joints, a cracked crown or lifted flashing seals the water in. The correct order is: inspect, repoint and repair, then waterproof the sound masonry.