Oh yeah, one more thing.
If you do exhaust and fresh air venting, which is a must for painting you need to oversize your heater a bunch or your shop will be cold in about 3 minutes after you kick on the fan. Most heaters are rated in BTU's, and a ton of heat is 12000 BTU's. To heat you can do a decent job with 4-800 ft/ton depending on insulation and ceiling height. If you vent it heavily the amount of heat needed to hold temperature even a little while goes up dramatically. I would try to get 40-60K btu heater at least if you want to keep the shop warm for more than a few minutes if it is very cold outside at all. My 40K unit will heat my 1200ft shop in a half hour or so if outside temp is 20-30 degrees. Then I can idle it back to low and hold temperature....or keep it throttled up if vented or a door is open.