as others have said - it can be done - but it's really important to check the temperature of the parts you're painting.
The paint is going to assume the temperature of the part you're painting, it doesn't care what the air temperature is.
It can take a real long time for cast iron to come up to room temperature.
If you've got a jet type heater, blast the part directly with heat from that - as long as the air's moving over them, they should come up to temperature quickly.
(of course - you'd turn that off before painting)
If there's no circulation, it'll take forever for the cast to warm up. At a minimum, just use a house fan to keep the air circulating over the parts.