Apply some ethylene glycol to the wood, especially the ends and knots.
About three applications a week apart should suffice. This will act as
a humectant, keeping the wood from drying too fast. Used antifreeze is
a cheap, available source. Once you have done so, the only coating that
will work is a polyurethane. Steel fasteners will rust more easily in
wood so treated, so use zinc-coated, stainless steel or brass
fasteners.-Jitney