Other alternatives are:
1) Propane catalytic heater: Advantage - no open flame and good radiant heat. Disadvantage - pretty darned expensive to run.
2) Kerosene heater (standard flame or "jet engine" type): Advantage - fuel is more economical than propane. A 20,000 to 24,000 btu heater will do a good job in 500-700 square foot garage. Disadvantage - Open flame and all the cautions that go with it.
3) Ceramic Heater(s). Advantage - Lower current draw than the open coil space heaters. Fairly economic to run. Disadvantage - Fairly limited in output but do put out a surprising amout of heat.
I have been using a combination of 2 and 3 for years. On non-frigid days I can usually fire up the kero heater and our small ceramic cube to get the
630 sq ft garage to comfort level and turn off the kerosene heater. Frigid days (20 degrees or below and wind). I run both. Obviously the open flame requires some precautions.
- No gasoline in the garage - including the cars (they go outside)
- Heater is separated from the woodworking by about a stall width
- Keep the heater clean
- Turn the heater off several minutes before you open any solvent or flammable finish cans.
- Keep the sawdust cleaned up and away from the heater.
If you have no heat source you might give gas more thought. You can often pick up a recycled residential force air furnace for very little money ($25-$100). If you space it off of the floor a foot or two you can overcome a lot of the flame concerns. A residential furnace will heat up a 600 - 800 square foot garage space in minutes. A cousin just did this and it is great!