Advice I had found on line suggested that I should treat the exterior of the tube first and seal it with varnish. Only after it was so sealed should I blacken the inside of the tube, the reason being that if the black got to bare wood on the outside it would not be possible to clean up. I can’t remember how I stumbled on boiled linseed oil (BLO), but I have ever since become its biggest fan. I treated the outside of the tube with several treatments of BLO, which added a beautiful yellow-orange tint to the wood and greatly enhanced the visibility of its grain. To at least slow down warping of the wood through moisture and also to cut the sun’s blackening effect on BLO, I gave the exterior of the tube two coats of spar varnish. Afterward someone suggested that varnish is hard to maintain, and various scratches since have convinced me of the truth of that. Still, the shine of varnish has its own appeal, and I have no real regrets for using it.