I've had great success using Tung Oil on my teak plywood
bulkheads and solid teak trim. 9 years ago when we bought the boat the bulkheads were black with mildew and soot from an old wood stove. After 6 or so passes with scrubbing with green abrasive sponge soaked in various combinations of thinner, acetone, Murphy's soap and water
works great BTW- and very light sanding
with 150 and 220 grit sandpaper, I got rid of all the black junk and could see a nice layer of teak veneer. Then put on 6 or 7 coats of tung oil - rubbing it down after dry each time, and got a very nice high sheen surface. Not glossy shinny like varnish, but a very nice hand rubbed furniture grade wood finish. Which has more or less lasted 9 years now. Also recently have starting experimenting with Deks oil high gloss finish, and various brands of teak oil. All have maintained a nice matted to glossy finish, depending on how much effort I put in each year. (Usually one or two,passes) For annual maintenance
, usually I wipe down all the wood with thinner and white towel then add a new coat.
Ideally you would need 6 or 7 coats of Tung Oil, or other penetrating oil such as teak oil, with a light sanding
after maybe 5 coats to produce a very nice furniture grade finish - I think perhaps it will last longer than varnish- certainly with less stripping and refinishing than varnish, which is why i opted not to varnish the interior in the beginning.
Like someone else said - BE REAL Careful with sanding. Modern teak plywood
is very skimpy on the veneer- now usually nothing more than a surface layer for looks, certainly nothing like the 1/16 th inch mahogy or teak veneers that used to be available in plywood in the 60's and 70's. So any hard sanding with even 100 grit sandpaper runs the risk of taking off the "teak" and leaving the underlayment exposed.