Missouri masonry receives wind-driven rain, humidity, and freeze-thaw exposure. Compatible chimney waterproofing may reduce liquid-water absorption while allowing water vapor to escape. That is different from coating the chimney with paint or a non-breathable film.
Waterproofing does not rebuild a failed crown, replace missing mortar, correct flashing, or stabilize loose brick. Begin with a chimney inspection and complete necessary chimney masonry repair, chimney crown repair, or chimney flashing repair. The masonry may need appropriate drying or curing time before treatment.
Ask what product category is proposed, whether it is vapor-permeable, how surfaces will be prepared, and what conditions it cannot fix.