How do you remove wood varnish from the window glass?
First of all, if the varnish is dry, you could try to scrape it off
with a razor blade.
Another method you can try is to use paint thinner or stripper.
Apply the thinner/stripper to the varnish you want to remove from
the glass, and let it sit for 30-45 minutes. Then wipe off the
excess and finish by scraping the remaining softened residue with a
razor blade. Follow that by cleaning the window with Bon
Ami (available for purchase online; see the Related link
below) on a clean cloth, using a circular motion. Rinse, then mix
1/2 cup white vinegar to 1 gallon warm water, and finish cleaning
for a sparkling clean window.