It was somewhere outside Barstow when bkr
wrote:

how do you prevent your varnish from
kicking to a hard molasses substance.

Speaking personally, I do it by not using polyurethane. I don't use
many varnish or varnish-like materials and those I do use are spirit
varnishes (plant resins in alcohol) or shellac. Plain solvent
varnishes like this don't suffer from the skinning problem. The one
poly I do use is Patina, a gel. This comes in fairly small tins and I
generally use them until they skin badly, then dump them. The tin size
is such that this isn't a big waste - about once a year.