Martyn, Ive been painting professionaly for about 8 years now, prior to that i used to spray outside but with the weather here that was no good. I had my loft converted into a workshop and the painting is done against the back wall which has a large extraction fan to the outside. In the winter (and sometimes summer
) i let the room warm ip for an hour or so
Regarding temperatures i would guess at a minimum of around 15-20 c. Colder than that, the air holds more moisture which condenses onto the model which causes blooming and inconsistant finishes. The model and paint should also be up to a reasonable temperature. Im not sure about the science of it all but experience has taught me this.
Hope this makes sense
Warren