I have a basement window with a metal frame fitted into a cinder block wall. The sill is crumbling and I'll find the right masonry product to repair that but the seal/transition from the frame to the wall was also in bad shape. It wasn't hard enough to be mortar and seemed too brittle and clumpy to be caulk. It felt like hardened glazing compound. I'm unsure what to reseal it with. There is a gap of about 1/4". Any thoughts?
Urethane or 100% silicone caulk. Urethane for the best bond and toughness, silicone if you can't find urethane. Silicone is not paintable.
Clean up all loose material first. Press in foam rope as a backer if needed, leaving room for at least 1/4" of caulk.