It warms up nicely with the lights and heater on in the garage and the varnish flows well. But the temp. will dip just below freezing during the drying time, should I be concerned?
I think you may be able to varnish. Maybe keep the heat down some so to avoid an extreme range?. I recently did some varnishing in the high 40's F or low 50's and it worked fine. I thinned the stuff a little.
I have had problems once with dew on new varnish. It left the varnish cloudy, but all I had to do is scuff and recoat.