I don't have any information on a "granta playwood kayck." Sorry. Never heard of Granta.

If the kayak just has scratches in the varnish layer, all you have to do is sand the varnish with some medium or fine sandpaper, clean the dust off, and apply a new coat or two of varnish. This sanding is just to help the new coat of varnish to stick to the old coat, so it should be done to the entire kayak, not just the problem areas. Polyurethane would probably work fine too.

-Dan