[V]

the safest for working with chemicals would be hot water baseboard heater. anything else will be a fire hazard.
If an attached garage, you could possibly add a loop to an existing zone, otherwise run an additional zone, but both may require a furnace upgrade.

a commercial unit would be something to look into since they might have fire safe designs

I use a dual burner propane heater, but i warm up the garage prior to doing any paint work. then shut it all off and paint. I then let the temp slowly drop(good insulation) and when it gets really down or when i can, I open garage to vent it all, then reheat since the fumes have cleared.