So my first question is basically, how do you prevent your varnish from
kicking to a hard molasses substance.

Don't let it come in contact with oxygen.

What should I do once I've
separated the actual amount I want to use from the main can?

Either displace the oxygen with "bloxigen" or similar, or transfer the
remaining finish to a sealed container about the same size a the volume of
finish to be stored.

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?

Less material being applied at a time. Drips will be less of a problem and
the resilting finish will be more even, but it will take more passes (time)
to get a similar "build".


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?

If it give you results with which you are happy, stick with it.

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?

No. When varnish cures, it undergoes a chemical reaction with oxygen which
is (for the most part) unreversable. This is unlike shellac or lacquer which
"cures" by evaporation and can be later "melted" with the original solvent.

If you have a skin on the varnish, you can strain out the solids and get
some more milage out of it, but that is probably a bad practice for other
than "quick & =dirty" work.

Steve