You can contact http://www.yachtpaint.com/usa/default.asp and ask them.  They answered me in less than 24 hours with every question I had for them.  GREAT customer service, and the makers of Interlux.

50 degrees is the posted minimum temp.

I would not expect desirable results from my boat and finishing job being exposed to 20 degree temps.

Best time to do this is early Spring before the pollen is flying around.

The fumes from this stuff is intense, even wearing a respirator, which you will be wearing, you need some sort of ventilation, even if your garage is a 2 car garage.

hello i have kind of the same problem you may want to try getting a long portible electric base heater . place it in side the boat . an other product  maybe  japan dryer that you could put  in the varnish  to help speed up the drying . i my self would try  the heater if  i had  one i wound up bringing the boat in the house .

I would stay away from using a space heater.  Varnish has far too many phenols in it and the vapors in a closed space could be a fire hazard.  Wait until spring.  If your November temps, like mine, are in the 30's and 40's, you're not paddling until April anyway... so what's the rush?  My kayak is on hold until March/April... I plan on doing prepwork this winter, but won't touch epoxy or paint/varnish until spring. 

 My $0.02. 

~Chris

Here in Alaska we would have about a 1 month building season if we waited for spring.  Why not use the good ole clamp lights to warm up the project?  We had two setup in the shop about 40F and it brought the surface temp up to 80F.  No fan to create dust either.  This is my plan for the winter, is there a problem with that?