Here are two tips on making your garage warmer IF it is unheated and unattached from a house:
1) Let the vehicle sit outside in the sun and warm up the paint. This may be a "problem" if it is a windy day, as outside debris (fall leaves and grass or bugs) from the wind can raise havoc a clean car and will reduce the radiant heating effect on the paint.
It is best if you can work on the vehicle in the warmest part of the day (mid-afternoon in the winter).
2) Let the car heat up idling in the driveway with the heater and fan turned on to HIGH for about 10-15 minutes, then drive it into said garage. The radiant heat from the engine alone may raise the temperature in the garage by 10°F, depending on the size of the garage and the size of the engine. Yes, it is a waste of "fossil-bio fuels" and contributes to "global warming".
I would still have a thermometer in the garage placed about a foot from the floor. Why so far down? Warm air rises and the coldest part of your garage is at the floor. If it is too cold near the floor level (IE below the 50°F) you may have some drying/curing problems with the coating on the lower rocker panels on the vehicle in the garage.
Tip number 3) Use a hair dryer to heat up a cool panel. Desperate times require desperate measures.