The rubber on a car window can become cracked or chipped over time. Rubber on car windows usually dries out as a result of sun exposure and poor maintenance. Once the rubber dries out, it tends to shrink. This can cause irreversible damage to the rubber. Fortunately, depending on the extent of the damage, it can be repaired using a special sealant.
Wash the rubber with soap and water using a sponge.
Dry the area with a lint-free towel.
Apply a bead of urethane sealer to the window rubber and smooth it out over the rubber with an applicator pad. Make sure that the urethane seals any cracks or covers any chipped areas in the rubber.
Allow the urethane to dry (called "curing") overnight before exposing it to any sunlight (or elements in general).
