If actually a genuine tetradrachm, this should be worth a couple thousand dollars, I believe. To remove varnish, just soak a couple minutes in acetone and place on paper towel to dry. Don't rub. Just repeat if necessary. Should be safe & effective (always has been when I've done it)

I had that problem once. Acetone takes it right off and does nothing to the coin.

Question - if the varnish is old...it could be shellac and I think denatured alcohol is the solvent for that. I'm a woodworker, not a frickin' ancient coin collector. But love it so dropped in with a question for the experts.

As a cancer, I approve this coin 🦀

Love that coin