If your teak looks like this
Or this, it's time to revarnish.
First, sand off all the old varnish.
Keep sanding until your teak looks clean of all varnish.
Then sand teak with 220+ grit sandpaper until smooth.
Wipe down the teak with acetone, then mix West System's 207 and 105. Apply in a thin coat
Allow 12 hours to dry and apply second coat of West System 207/105.
Allow epoxy 12 hours to dry, then apply thin coat of Epifanes Woodfinish Gloss with 10% dilution of Brush Thinner.
Try to avoid drips! Apply second coat 12-18 hours later. If you can't apply within 24 hours, do a fine sand (220+) until varnish is cloudy before the next coat
Your finished product should look and feel smooth. This system should last 6 months to 1 year, depending on your location. Be sure to check, because you'll be redoing epoxy if you let it go too long!