Pete, I might be jumping in on the thread but I've just turned a small bowl from an old peach tree and it's full of beetle or bora holes/tunnels. I've tried a few methods of filling them in but haven't had too much success. Today I used pva glue mixed with some light coloured saw dust. Tends to shrink, takes ages to dry & often goes very dark!You don't need to limit yourself to coffee grounds. Any powder with a shade or colour you like is fair game. Old spices like mustard, curry etc for example. Semi precious stone dust, brass fillings, just about anything you can think of have been played with.
Pete
Oh, in case you were wondering, l exposed the part where the beetle was and he was flat out chewing his way around! What would you normally do with this? I'm going to fill in the inside holes and leave the outside as is.