When it comes to sealing, chimneys tend to be more of a forgotten application. That is until you come home one day to find the chimney leaking! Since chimneys are constantly exposed to the elements like wind driven rain, sun, snow, sleet and hail, they are at risk. In comparison to the cost of replacing a chimney, using a waterproofing sealer will pay off dividends in the long run. The best type of sealer to use on a chimney, both brick, stone or masonry based, is of a silane/siloxane chemistry. Silane/Siloxane products are penetrating and will leave no visible sheen or gloss. They are excellent at repelling water, preventing mold and mildew build up and preventing unsightly efflorescence. And the best part is after treatment you do not need to worry about your chimney ever leaking!
One of our favorite silane/siloxane products that we typically recommend on this site is the Ghostshield 8500. We love this product because it is available in an ultra concentrated formulation which saves on shipping cost and it is a lot easier to move around and manage in comparison to a five gallon bucket. The price point is also on par when broken down it comes in around $40 dollars a gallon.