ORIGINAL: 08gtrking
I was thinking waterbased urathane the interior and outside. im not 100 % sure if that will water proof it so ask some more people see what they tell you.
WB Polyurethane is water proof once cured, it even stand methanol and low nitro quite well. The best about it is almost no smell and being able to clean your brushes with warm water. Like all varnishes it is sensible to long UV exposure.