I removed the original bus windows to clean and reseal them. Most of the original weatherstripping has been replaced by caulk. I'd like to replace it with something similar, since so far it has been effective in preventing leaks, and it still allowed me to remove the windows, which might be necessary in the future.
Here's a picture:
It's gray, still flexible, and not as messy as the tarry black sealant that lined the floors. Any idea what kind it is?