Varnishing Teak Veneer
I have usually lightly sanded with 220 grit, tacked down the plywood, wiped it off with alcohol to remove natural oils and then applied a first coat of 1/2 varnish, 1/4 penetrol and 1/4 terpentine. That will raise the grain slightly. Then I lightly sand with 220 again and use straight varnish. When I have a big sheet of plywood to do, I use a roller to apply the varnish and then quickly tip it off. If you have the luxury of being able to keep the plywood horizontal you can avoid runs and apply a bit more millage with each coat. I am a big fan of allowing each coat to thoroughly cure although I will often use a quick drying varnish to build up base coats. I typically do not try to get the varnish very level until the 5th or 6th coats and then once sanded level I typically apply a two or three topcoats.
The other thing that I have seen people doing lately but personally have not tried on anything this big, is to do a couple coats of epoxy for the first couple coats and then sand and put a couple coats of varnish over it. Prep is the same as for varnish. The idea sounds good and I tried it on some brackets that I made last year when I installed a new stove.
One last point about teak veneered plywood. There are several grades. For a few dollars more I would try get a ''deck grade'' which has a thicker face veneer.