I am a registered August-West Chimney sweep in Cheyenne, WY. Chimneys out here take a real beating due to the extreme temperature changes, and high winds. But, the real culprit is moisture! Masonry chimneys, because they extend well above the roof line, bear the brunt of the elements. Waterproofing your chimney is a very good idea. It protects the mortar joints from moisture entry, and makes a chimney repair last a lot longer.
A product is available called "Chimney Saver" and is only available through a registered chimney sweep. It is available in both solvent or water base. It comes with a 10-year warranty, as opposed to Thompson's Water Seal that has to be replaced on an annual basis. Chimney Saver will not alter the surface appearance, and here's the good news - it does not allow water to penetrate, but allows the chimney to breathe, thus allowing humidity to escape. Other waterproofers cannot do this. Your chimney repair person is absolutely correct, the masonry must be able to breathe because of the moisture that is created in the combustion process. Chimney Saver is also great for preserving your entire house, if it is a masonry structure.
If you cannot find this product in your locality, please contact:
Above All Chimney & Fireplace Services c/o Rich or Buck 3529 Boston Road Cheyenne, WY 82001 (307) 634-8722 or simply reply back to me.