Buzz,
Mark's website (Marklarsongunart.com) describes his finishes under FAQs. He uses a variety of finishes but an oil/urethane mix most often, which holds up but has the look of a traditional oil finish. I think he is constantly perfecting his own tricks of the trade for concealing or minimizing the visibility of the seam. Although Dave Wills in Montgomery, AL. made the extensions for me, on which Mark did his magic grain and figure blending, Mark's website indicates that he may now also be able to provide the wood extensions.
