I have a couple of different cans of varnish that I opened and dispensed into smaller containers for use. Now the varnish has hardened in all the various conatiners, whether sealed or not. I know it has to do with reaction to the air, but I don't know how to prevent it. I've seen a couple threads that I think suggested adding some other chemical to the can that wouldn't mix with the varnish but would displace the air.
So my first question is basically, how do you prevent your varnish from kicking to a hard molasses substance. What should I do once I've separated the actual amount I want to use from the main can?
My second question has to do with thinning. I've read somewhere to thin with mineral spirits or something similar. What are the benefits of doing this? I've used the varnish straight and love the finish and I haven't had any real difficulty in application such as bubbling or anything. Are there benefits to thinning that I'm not aware of?
My final question (for now) also has to do with thinning. Is it possible to thing varnish after it has become the molasses substance I described in my first problem?
Thanks for any and all pointers (especially links to places I can do more research).