It takes a lot of sanding and effort to get back to "green teak", and even if you varnish it, it soon loses that original appearance. Too much sanding = too much loss of wood.
On my side nameboards I sanded smooth but not totally "green",and used a diluted teak stain, quite dilute, applied several times to get the degree of color I wanted. Worked a treat, under Cetol TGL Plus Gloss, my choice of varnish.
Left untreated, your teak will develop ridges as soft material between harder material wears away, just from weathering. Don`t leave it untreated, I have some areas a PO left, and the ridging is such that I`m not prepared to sand completely smooth because of the resultant loss of wood, I just let it happen gradually as I maintain the finish I applied.
If you apply an oil, and keep applying it as soon as the surface looks a little "dry", that may be enough. The initial saturation takes some time, but after that it`s just maintenance. Where I keep just an oiled surface I use Deks Olje No.1, but mostly I add the related gloss product No.2 on top. Their system rarely requires hard sanding or complete removal of the old surface,unlike varnish type products. How do I know? I`m doing my annual revival, right now.