How does chimney waterproofing actually work?
The pro starts by looking at the masonry itself, because waterproofing seals a chimney in whatever condition it is in. Cracked mortar joints, spalled brick faces, and crown gaps get repaired first; sealing over damage locks moisture inside. Once the surface is sound and dry, the pro cleans off dirt, efflorescence, and any old coatings, then masks the roofline and nearby plants. The repellent itself is typically a siloxane- or silane-based product that soaks into the brick rather than forming a film, so the masonry can still breathe. It goes on with a low-pressure sprayer in a flood coat, working from the bottom of the chimney up so runoff saturates the wall evenly.
Most products call for two coats applied wet-on-wet, and the pro will watch the weather; the masonry needs to be dry going in and rain-free for a day or so afterward while the repellent cures. Expect the work to include the whole exposed stack above the roofline, sometimes down to the shoulder if the chimney runs up an exterior wall. Good documentation matters here because the treatment is invisible when done. Ask for the product name and data sheet, before-and-after photos, notes on any masonry repairs made first, and the terms of the applicator or manufacturer warranty, including how long the treatment is expected to last and what would void it.