Do not use varnish. Do not use shellac. Do not use anything except first-class linseed oilwith, for the first coats, a touch of pure, natural turpentine. And polish the surface with the method known as "boning" (effectively using a scraper with a trailing action to smooth down the wood fibers).
I described how to do this in detail in a multi-part thread on restoring an Argentino Rolling Block. Which seems to have gone AWOL. As it took me weeks* the first time, I do not feel inclined to do it all again. Please search for "boning" or "treacle rifle", and you should find a couple of before/after photos
Patrick
*correction: months!