Here's West's page on undercoating with epoxy prior to varnishing, click Epoxy Techniques & Materials, then Varnish Over Epoxy...
http://www.westsystem.com/frames/tier1/usesforepoxy.htm
They say this results in the finish lasting longer, because the wood surface becomes stabilized and the varnish will crack less from expansion and contraction.
I've done this in the last set of ash outwales I made - they're L-shaped in cross section to fit over the sheer and inwale. Sealed inside and out with two coats of epoxy, including inside the screw holes which hold the outwale on. Three coats of good varnish were applied after the outwale was fastened on. The joint line between outwale and hull was left open and unvarnished (masked off) to allow the inner surfaces to breathe and dry if water gets in, which it will with waves and rain.
So this should result in totally waterproofed, long-lasting outwales, unless they're scratched or chipped, but some touchup when necessary should be easy enough. Other outwales I made weren't sealed in epoxy and years later started to blacken where the end grain and screw holes would let in water. The epoxy and varnish combination gives a deep-varnished look which is very glossy , some like it, and some don't.
I don't know when I'll be able to get back to you on just how durable this epoxy/varnish combo actually is, I'll post here again in maybe 10-20 years, we'll have to watch closely for when those first signs of blackening start to appear.
Rick