1: never varnish teak
the natural oils wont let it stick for long.
One way to get a non-slip finish on varnish
is to build up you coats as normal and then when the penultimate coat is tacky sprinkle it with sugar. When dry wash off the sugar and apply the top coat, you'll have a nice glossy looking finish with a slight stipple - works! Used to be used on dinghy
side decks when they were varnished apparently.