A BIG NO ON EPOXY! Will lift and crack, get water underneath, and deteriorate the wood under the epoxy. That's the worst thing you can put on Teak.
Varnish for a high-gloss finish, CAPTAINS or SCHOONER varnish. They are the top brands, will provide a delightful finish but will require periodic re-coating. If you let it go too long and get voids in the finish, you have to strip and start over.
It takes about 8 coats to get a good finish. I used to get 10-12 on them. First couple coats should be thinned with turps, then lightly sand with 220 grit and after every couple of coats. Tack-rag it and re-coat. You need a premium brush, like a hamilton or something similar.
For maintaining the finish, light sand with 220, apply a couple of coats. They can go on top of each other at 12 hour intervals.
I had a Monk designed sailer in the NW with teak EVERYWHERE - rails, coamings, decks floors, hatches, cove mouldings -- acres of teak. The hull was red and yellow cedar, spruce spars, every piece of hardware was bronze. She was quite a show boat. A shipwright on Lake Union had built her for himself, took him 7 years. Wifey would never go out so it sat and got deteriorated. I bought it and restored it, was a gorgeous little vessel.
The decks were teak too, they got Deks Olja, all the teak trim got Schooner Varnish.
You want a top coat that is durable to keep sun and water off, but soft enough to expand and contract with the wood, and easy to sand down and recoat.
If it has already turned gray, scrub it with a stiff brush and a good teak cleaner, then sand with 150-180 grit, finish with 220. Teak is VERY hard, but still wood nonetheless. It is also very oily, and will dull tools quickly. I know, I've worked with plenty of it building and restoring wooden boats.