Repairing Weather Stripping...
To fix superficial surface cracks as well as small gaps in the weather stripping I have used "black rubber sealant " by PermaTex (it comes in an orange tube). It is a fairly low viscosity goop that has a fast dissipating solvent so that it dries fairly quickly. It sticks to rubber (obviously) and what remains is very soft & pliable, which makes it excellent for this use. Do not confuse this with silicone sealant.
The best "tool" to use is your finger -- I just use some disposable nitrile gloves. Where the weather stripping is cracking on the surface, put a small blob of this stuff on your finger and rub it into the surface. You'll need to work quickly as this stuff does dry fast; try to leave as smooth of a surface as possible when you first smear it on. For small gaps, you can build up a few coats and form it with each successive buildup. This method works surprisingly well.