Another advocate for heat gun and scraper. This is based on several varnish-months (VM) of experience. A varnish-month is the expenditure of 170 hours of labor directed at removal, surface prep and reapplication. That can happen when you have a boat like this for 48 years. I have had the best luck with Flagship.
Another option - to reduce VM's - after they're stripped, just slush 'em down with old motor oil as they did on Gloucester fishing schooners. Then reapply as needed from your bosun's chair.