ChimneySaver is the industries' top choice for waterproofing. It actually changes the polarity of the chimney surface to the same as water. This makes the masonry repel water like two negative ends of a magnet. Since it contains almost no solids the chimney can still breathe. This is important because moisture inside the chimney can still wick through to the exterior whereas solids would trap the moisture leading to freeze/thaw damage.
To apply ChimneySaver it's important to use a sprayer with a variable spray head from a straight stream to a flat fan spray. You should start at the base and work your way up. When runoff starts to appear 6" below the spray pattern it's time to move the spray head up. Coverage ranges for a brick wall are 100 – 200 square feet per gallon, concrete 75 - 125, concrete block 50 - 125, stucco and limestone 75 - 150.
The most important part of applying ChimneySaver is the prep work before hand. Overspray and spills can stain shingles, paint, windows, siding, etc. It's important to lay down Sorbent Pads at the base of the chimney. Don't spray on a windy day and protect adjacent parts of the structure with tarps. Cover, protect or move anything that would be on the ground near the chimney base or where overspray could drift.
From the homeowners point of view they may have dealt with a stained or leaking chimney. Since they can't see ChimneySaver in action unless it's raining, it's very important to make sure they know it won't change the surface or appearance of the brick. They need to understand the value of the application. They are purchasing a chimney "coating" with the following benefits: stain-resistance, leak-proofing, better smelling (less moisture for stinky smells) all while retaining the same visual appeal. With a ten year warranty they won't have to think about it for another 10 years and as a side affect their chimney will be preserved with less future maintenance.