As long as you are doing some tests, try adding a small amount of boiled linseed oil to one batch of your test varnish

If you are using half an ounce of varnish or less for each test, get an eyedropper to measure the oil.

Somewhere between 5 % and 10 % should increase the flexibility of the finish. Either that or it won't work at all.

PGJ

Ya know if you used all natural walrus skins instead of synthetics you wouldn't have this problem--you'd have some other problem. :)