I am afraid I cant give you any more until I have soaked my log for a few weeks!
I hope the epoxy will keep the log intact especially the bark but, I think it would need several more coats to guarantee a complete seal. The bark makes it very difficult to fill every nook & cranny however the log does have a very good coating.
Polyurethane should of given you a good seal too - Bondaseal G 4 is a good product.
I wrote a bit here viewtopic.php?f=21&t=13510
about making backgrounds.
I buy my epoxy wholesale in 4.5lt cans 10 at a time but, it is still expensive
I will now give some thought to how to fix the log in place, I have a few options like making up two perspex strips about 75mm wide that I can fix into the log ends & clamp to the top of the tank. This would enable me to easily remove the log for "maintenance"!!
I doubt if the log will sink even after soaking as any water ingress will be very slow?
However I want to see what happens after a few week in the water butt....