I just dip a rag into a can of polyurethane and give the barrel channel a light coat. If there are areas I can't reach with the rag I'll use whatever tool seems appropriate. Cotton swabs work well for tiny areas.
Whatever you use it needs to be a truly waterproof finish and form a barrier OVER the SURFACE of the wood. Any wood grain left exposed above the surface of the finish {as would be normal with a "light coat"} will allow the wood to absorb water and water vapor like a sponge.
