I can safely assume that the window will be painted again after the gunk comes off? What will be used for? An art piece for the garden?
From my previous life, when I was doing a bunch of restoration, this product was my friend. It can be shaped, sand, paint, stain, and depending on which product line you chose, its even structural. Let me introduce to you, my old friend, Abatron.
https://www.abatron.com/product-category/wood-restoration-maintenance/ if wood has rotten away or need to add a chunk that's missing. This stuff aint no Bondo like some people think. Its not cheap. Amazon may carry it?
The US forestry uses it on repairs in their remote structures way out in the sticks. I uses it on my parent's 1929 Spanish to repair some of the structural pieces under the house because I was too lazy, and too costly to tear the house apart. Its been 10 years and it hasn't changed still solid as a rock.
Yep, Gogar is right, I would have it dipped.