Window box leaks

[Cybo]

what a relief to find this site. several months ago I began restoring an 81 excella. Phew!

Inner wall panels are down and I've got leaks about the windows. not sure what was used back then to seal around window frames e.g. white substance, dark and cracked at leaks. I need to stem the leaks.

Question: Is Tempro suitable? If no, what should be used?

Thanks very much.