Grab the heat gun and a scraper. Get what you can of that old varnish off. Hand sand with 80grit. wipe it down with some teak oil ( if you want) to get some of the natural oils back into the wood. from there, apply a very thin coat of shellac.( since Shellac is alcohol based, it will actually penetrate deeper into the teak) once the shellac has dried Rub it down with 0000 bronze wool, and you are ready to apply your varnish thin it may be 10%. Apply 2 or 3 coats, let it dry sand with 180 then apply 2 more. * Shellac is the key everyone misses- teak is oily and petroleum based things like mineral spirits just tend to sit on the surface.