Anyone who varnishes paintings understands the frustration of ruined paintbrushes. When varnishing a painting one must use a soft bristled, new condition paintbrush to avoid streaking. But once you varnish a painting that paintbrush gets ruined, no matter how many times you rinse it out, right? I have a solution.
I have discovered that if I rinse my paintbrush thoroughly then place the brush in some water the bristles will not stiffen, or harden. Yes, you need to keep the brush immersed in water at all times when not in use. Designate your brush/brushes for varnishing and only use them for that. If you keep the brushes in water while you are not using them they will be soft and like new every time you need them for varnishing.
Because you are keeping them immersed in water make sure you select a paintbrush that does NOT have a wood handle. I keep my varnishing brushes in a jar of water in my workspace. When I need to varnish I simply select my brush, dry it with a towel and begin varnishing.
While you may still need to replace your varnishing brushes periodically, this method should greatly extend the amount of time you will be able to varnish with the brush. It's the ONLY tried and true method I have discovered that truly works. Give it a try!