Water penetration is the single greatest cause of deterioration and damage to chimneys. Spalling bricks, leaks, efflorescence staining, and cracks from a freeze thaw cycle are all problems that can result from a chimney's natural tendency to absorb moisture. Sealing your chimney with a vapor permeable water repellent will prevent the chimney from absorbing excessive moisture yet still allow the chimney to "breath" so water vapors can escape from the structure. The application of a water repellent can greatly add life to your chimney and prevent any water related problems from occurring in the future.
Spalling bricks, deteriorating mortar joints, and moss growth can all result due to excess water absorption. Deterioration of this nature can be due to how much sunlight the chimney receives. It can also be a result of the type of brick that was used to construct the chimney.
Unsightly efflorescence staining and leeching salts due to excessive water absorption.