Cut paste:
I inherited a yew tree which had been growing for a long time. I cut a huge piece of the trunk off and then let it sit out in the wind and rain for maybe 10-15 years. This year I finally started doing something with it, and chopped off a whole lot more - there was some discussion about this tree a couple of months ago.
Water had seeped down into the open cut on the trunk, rotted a little, then more fell in from rain, watering, etc. I don't know if it is coincidence or not, but there was a huge chunk of bark dead and gone down the side. I have spent several hours digging out the wood trying to get to the bottom of the rotted dead spot so that water won't collect there and keep on rotting. I haven't reached the bottom yet. The chunk I have cut out is about 6"/15 cm.
I think the rotten part is actually going to look real good, probably better than I could carve, but I don't know that this is what I would count on.
Locally, our group mostly uses Elmer's wood glue for cut paste, some full strength and some cut 50% with water. It looks ugly the first time it rains, but after a couple of weeks goes transparent. I don't know what would be a similar product in Europe or elsewhere.