Waterproofing your chimney is absolutely the best thing you can do for exposed masonry. It can literally add years of protection to your exterior brick/masonry. Chimsaver is the best Water Repellent that I've used in 35+ years in the industry. It helps protect the crown, premature spalling brick, and efflorescence. (white stain on masonry). Also, it allows the brick/ masonry to breathe so moisture is not trapped.