With some things, there isn't a "quick fix."
The first thing I think when someone says that is half the chimmey is falling over. In that case, a non-slippery canvas tarp and tuckpointing is the only option.
Repainting the entire chimmy with an exterior uv-grade expoxy paint is the best long-term sealant, provided the flashing is good.
(edit-here is one pro company...no experience with any of the products...
PROSOCO, Inc.)