The problems of an open area is the amount of heat needed to heat it. If you have a garage open to the rafters, there isn't a portable electric heater large enough to heat it. I know I have tried.

Catalytic kerosene heaters and stoves that are rated for residential use are relatively safe. They are the primary means of heat and cooking for many people in the far east (Japan, Korea). You must use K-1 water clear kerosene. If you have to use solvents (those more volatile than mineral spirits) just shut off the heater.

Typically, if I use the heater it is on for an hour and off for 3 (good insulation helps here). Insulation is most important in the ceiling.