Good spar varnish has tung oil as its major component. Do not get polyurethane!

Contact Messing About in boats with a letter. There are certainly readers there who have had your problem. The address is:

Messing About in Boats
29 Burley Street
Wenham, Mass   01985--1943.

Phoning Bob Hicks directly won't help and Bob is not on line. He will give the same advice I just did, so this is your best bet.