Hi Frankie, have you ever considered using Plasti Bond. It is similar to car bog and when I was a teenager my old man used it a lot as a filler on his timber and later fibre glass sailing boats and saw first hand how good the stuff is and also how easy it was to work with. No good if you are wanting to keep a timber look around the repair as it would need a solid paint to cover and hide things.

http://www.selleys.com.au/trade/buil...nd-heavy-duty/