It is recommended that brick chimneys be waterproofed. We use Chimney Saver® water repellent which is 100% vapor permeable, meaning it keeps water out but lets any moisture inside the chimney escape. This is a great product.
For chimney waterproofing to be fully effective, the chimney should have a flue cap, properly constructed crown free of defects, and the brickwork should be free of missing, cracked or spalled bricks. The mortar joints should be full and free of large cracks, and the flashing must be intact and properly installed. The bottom line, you can’t waterproof a defective chimney and expect to solve the problem.