I would use Nitromors to remove the varnish...makes it a very quick and easy job.

In case you havn't used it before, be cautious as it gets everywhere and of course use latex or similar gloves.

If its warm and the nitromors is tending to 'dry out' to quick, cover with cling film or ali foil to keep it 'working' longer.

Possibly with a dinghy sized job it might be worth applying with a mini roller.

Regds Nick