The order this is done in
Repair first, then treat. Open joints are repointed, a cracked crown is repaired or replaced, spalled units are cut out, and only then is a repellent applied to masonry that is sound and dry. Reversing that order is one of the more expensive mistakes available here, because a repellent has to weather off before the joints underneath it can be properly repointed.
Efflorescence, and what it is telling you
Efflorescence is the white powdery deposit that appears on chimney brick, on a chimney breast inside the house, or on masonry in an attic. It is salt from within the masonry, carried to the surface by water and left there when the water evaporates. It is not harmful in itself and it is not mold. What it is, reliably, is evidence: a path exists through that masonry and water has been using it. Cleaning it off without closing the path just clears the evidence.
Is efflorescence mold?
No. Efflorescence is a mineral deposit, usually white or grayish, and it brushes off dry. Mold is biological growth, more often dark or colored, and it appears where organic material has stayed wet. The two get confused because they show up in the same conditions, on a damp chimney breast or in an attic beside a leaking stack. If what is on the wall is mold, that is a different piece of work, and this business does both, which means nobody has to guess in order to sell you the one they are holding.
What waterproofing does not do
It does not stop a leak. If water is coming in through a cracked crown, an open flue, or lifted flashing, a repellent on the brick face changes nothing, because that water is not traveling through the masonry. It does not repair damage that has already happened, and it does not replace pointing. It reduces absorption on a sound stack so that the freeze and thaw cycle has less to work with, which is a smaller claim than the category usually makes. The National Park Service goes further and warns that waterproofing brick with chemicals sold for the purpose, or painting it with non permeable paint or other coatings, accelerates decay by trapping moisture behind the coating. That warning is the reason nothing goes onto masonry here until the masonry is sound and dry, and the reason the product used is vapor permeable rather than film forming.
The chimney and the water damage side
Water that a stack absorbs does not stop at the brick. A chimney that has been taking rain for years puts moisture into the chimney breast, into the framing beside it and into the attic, and what shows inside is damp plaster, staining and eventually mold instead of a dripping ceiling. That is restoration work, with meters and a written log, and it is the primary business here.
When to do it
Early spring in this market. Freeze and thaw damage becomes visible after the winter, the masonry gets repaired then, and treatment goes on afterwards while the weather is dry and before the storm season starts in June. That is a different calendar from the fall sweeping and inspection season, so the two are not bundled together here.