Varnishing teak needs a good cleanup before applying the coats. After using sand paper, use alcohol or acetone to clean the surfaces. Use two cotton clothes, one with alcohol or acetone and the other dry. Apply the solvent liberally with the first one and wipe with the dry one. Teak has a lot of oil in it which passes to he surface. A good
varnish will hold much better if you clean these oils which means a shiny surface.