SiNguyen

Hi Bonhe, yes that's true, bougies have very soft wood. But in this case, you don't have a choice. Your tree already has a huge gaping hole, and I am sure, a lot of rotted wood. At least you would need to remove all the rotted wood, down to the hard wood, before you can consider treating it with any wood preservative. Is that what you have in mind in doing? You should try to carve out some drainage in the deadwood so the water won't collect in the hollow spaces and rot out the wood even more.

Good luck!
Si~