Bricks and mortar are porous materials and act similarly to sponges. In times of heavy and steady rains, chimneys can fill up with water that can seep into the house. This can cause structural damage to the fireplace itself, or to the house. It can also lead to mold problems.
We have found that this will often still leak after being properly repaired, or even after a rain cap has been installed, and this is because waterproofing has not been done.
At Paramount/Rite-Way Masonry, we have been in this business a long time and have seen it all. We started using a product called Siloxane to water-seal bricks and mortar. It is a product that provides a long-lasting invisible coat to bricks and other masonry surfaces. It is applied by spraying or brushing it on until the brick and mortar start to repel the product. Siloxane allows the brick to breathe without letting water in.
The product we use has a 10-year guarantee. From our experience in using this product we have found this to be the case.