There are number of ways to "treat teak". Some like it the aged "silver gray". (do nothing but wash it with salt water and "joy". Some like the sanded look--and "Simco" teak protectorate or sealant will maintain that. Then there is the oiled (any Teak oil--but often the oils gather dirt and soot if in the air). There are several types of "Cetol"--Natural Teak or clear is probably what you want--the oil with multiple coats, and then put on the Sealant "Clear gloss wood finish". This is a hard finish, but not as hard as varnish. It can be touched up.
When cleaning teak, be very cautious of caustic brighteners. They dissolve the self grain in the wood, and you end up with groves in the grain--if you brush, with the brightener, go across the grain, not with the grain! Sanding is always a good way, but it does wear away the wood with time.
Generally stains are oil based and a good paint thinner will remove a considerable part of the stain, probably enough that some light sanding with 120, 240 and then 400. Don't go any less than the 400. Always sand with the grain! If you want to varnish, hit the teak with acetone before the first coat to remove any surface oil.
I use Tri sodium phosphate (same stuff to clean the waterway smile off the hull. You can get a lifetime supply of crystals at a paint or big box store. Make a saturated solution. I just did the inside of our boat's teak--no sanding necessary, just TSP and teak oil--looks like new.
We always came back to a quality varnish on our large cruising boats. The 62 foot ketch we cruised on for 4 years, we had to re-do the varnish every 6 to 8 weeks in the tropics. It was a several day job. There was over 100 sq feet of varnished teak--plus teak decks (only washed with salt water and Joy, scrubbed across the grain with a medium brush). On the Cal 46 we had done the whole Cetol thing, with 10 coats of the natural, and 3 coats of the finish in the PNW (only a couple of hand rails, and the entire toe rail, about 3" high and 1 1/2" wide)--it worked great there. But when we got in the Tropics, the UV light thru residual salt crystals began breaking it down--so we went back to multiple coats of varnish. Another way is using a clear epoxy, and then varnishing over it to give UV protection...problem is if you get moisture under the epoxy, it can lift, and make a defect. Also the wood moves, more than the epoxy can give...so you can end up with cracks.
Your choice.
