how deep/large is the damage?
About 4 feet long and it some spots 1/4 inch deep.
i have a good many crepe myrtles in my yard and have came to the conclusion that they're pretty much indestructable
You should let the tree heal itself. Black sealers actually inhibit a tree's healing.
I concur with the above comments. Crepes are fast healing plants. This is coming to the growth season for crepes anyway. It will heal itself.
Douglas and Eden are correct. While at a recent continuing education seminar, I heard a UGA professor discussing the tree coatings in further detail. To summarize, although the coating covers the wound initially to try and keep disease and insects from entering the wound, over time it becomes a coating that allows water to get behind it. And when water and moisture collect behind the coating, you develop rot. Once the rot starts, it can finish off a tree, as it will migrate deeper into the tree. Therefore, there is no need to paint any coating on the wound . Let the tree try and heal itself.
Let it heal itself, sealer will do no good