Official Luthiers Forum!

Hello,
I have a question about a project I'm working on for a client.
He asked me to do a few modification to a guitar body, nothing expensive or rare, just a recent Korean mass-produced guitar (so my conscience is clear ) and most of the work consists in remodeling the tremolo cavities for another tremolo unit, a top mounted Kahler in this case. So I had to fill in the spring cavity in the back of the guitar. I cut the plug from a well dried piece of wood, re-routed the cavity clean so everything is snug, glued in the plug (I had to hammer it in, so the fit was quite good) and I let it dry for weeks just to see how it reacts. Of course, it shrunk a bit because it was quite a large plug and now I have two questions:
1) What should I fill the small gaps that formed when the piece dried? I was thinking a two-part epoxy based filler, but maybe there's something better out there for this task.
2)What should I use to seal body, once I filled and sanded everything flat again to make sure that it doesn't happen again? I was thinking about CrystaLac sanding sealer, but that product is water based and I'm not sure that it will not react with subsequent layers of paint (probably a 2K polyurethane basecoat/clear coat).
Luckily, the client (or me) are not at all in a hurry with this guitar, and I just want to make sure that everything remains as stable as possible, because in Canada the humidity changes are not kind to instruments...

Thank you very much in advance for your advice!