I wouldn't have an open flame anywhere you're planning on doing anything with flammable chemicals, and a garage/shop is one of those. Every winter there are news stories about people who burned their garage down because they used flammable chemicals around an open flame.
An isolated (so the flame isn't in the shop), gas fired heating system is the only way to heat a garage or shop in my opinion. I can work on vehicles, paint something, or work with flammable materials without fear of blowing something up, even in the dead of winter.