Hansli,
Years ago I used the white of an egg (albumin) then I used egg gloss which you could by in tins from the chemist (builders used it to seal concrete swim / fish pools) then I used sanding sealer,then I used multiple coats of spar varnish each one cut back when dry.Wet sanding with stock oil always works.
BUT nothing really works like rubbed in oil and time and patience.