We had an old timbers and logs which were stored on the air. They lied down outside the house for several years.
When we decided to do the interior design in marine style - we wanted these logs. We did stairs from timber and beams of the stairs from the logs. Unfortunately I don't have photoes of these logs and timbers from the very beginning of our project, but firstly we cleaned them with carpenter's plane (we have electric one). After that we sealed up all the slits with special plastic wood filler. Then we took a grinding machine and sand the wood step-by-step with sanding paper by 80-100-250-500-2000 grit.
After achieving the necessary smothness of the wood we used wood preservative, after drying we coated wood by woodstain (two coats), and finally - three coats of furniture varnish. Between the layers of woodstain we sanded again with sand paper of high grit (2000 or even more if you have). It provides the super smothness of the varnish.