I was wondering what others are useing to heat up their shop for painting?

I live in kentucky and we have mild winters. I keep my shop warm with a karosene heater and 2 220 volt electric heat when I need to paint. but the electric heaters do all they can to get the shop warm. without useing both and the karosene heaters the shop gets really cold. what do you suggest?

the shop is a 40x60 with a 12' ceiling. all metal building.