I have recently been experimenting with mixing in a bit of Copal varnish (10% - 20%) to my Colophony/Linseed oil varnish, to add a bit more hardness/toughness and to make the varnish a bit more wear resistant.
The Copal varnish I am using is a very good one, made by James Groves. It is Very high in copal resin-content: 50% copal to 50% heat-polymerized linseed oil.
I am mixing it in cold to my own cooked varnish (50% Colophony to 50% Linseed Oil). This seems to have worked well on my test strips, and it does seem make the varnish a bit tougher.
However, I am concerned as to how this may fare in the long term - could it cause the varnish film to crack due to Copal's reputation for being brittle?
I'd love to hear from anybody who has tried something like this, or who has experimented with mixing a bit of copal into their Pine Resin/Colophony & Linseed Oil varnish.
Best wishes to all on this wonderful forum!