Paint does strange things at -5°c, at least that's the claim. I need to spray prime some miniatures and I need to do it outside. But the UK is having an unusually cold winter so far, and more snow than we normally experience this side of Christmas. I'm not too bothered by the cold myself - having plenty of blubber to keep me warm - but I wasn't sure if spray priming models in subzero temperatures was a good idea. With no change in the weather, and the cold predicted to last for at least another week, I decided "to hell with it" and lets see what happens when painting in sub zero temperatures. Before you all scream in horror I must add that I did do a little research on this matter by trawling various painting forums for advice and personal experiences.
Acrylic Spray Primers typically use volatile hydrocarbons such as propane, n-butane or isobutene (or a mix of these) as the propellant. These all have the advantage of a very low boiling point meaning they remain in a gaseous state at very low temperatures. In this state they evaporate off quickly, ideal for a paint carrying medium. However the propellant needs to be mixed to ensure a homogeneous mix which is why most cans contain a 'pea' and require a few seconds vigorous shaking before use.
At very low temperatures (-40ºC for Propane or -0.5ºC for butane) the propellant can cease to be a gas and achieving a homogeneous mix becomes very difficult. Low temperatures will also extend drying times and increasing the chances of 'external influences' ruining the finish. Some posters on painting forums suggest warming the spray can in hot water before taking it outside. However I would respectfully suggest that it is not a good idea to heat up a pressurised container filled with flammable gas! Instead keep the can indoors and let it reach room temperature before use. This is more than sufficient for the propellant to work in the way the manufacturer intended.
With the can and its contents at room temperature it is best to keep outside work to a minimum, bringing the miniatures and spray can inside as quickly as possible while the paint dries. If the miniatures get too cold when you bring then inside condensation can form and spoil the still drying primer. Of course this will be more of a problem if you are priming metal as apposed to plastic or resin minis.
So armed with this little bit of knowledge I tested the advice I had found on an old model just to see what the results were like. I normally spray between 8-12 inches from the model but I found that at the further end of that scale the paint applied grainy in texture. I closed the gap a little and the primer (in my case GW Black Spray) went on evenly and smoothly. I rushed the models back inside and the bulk of the drying took place there. It was a little smelly but nothing compared to spraying indoors.
When the primer had dried I found I needed to give them a second coat to catch some areas I had missed. The end result looks as good as normal and I can't see any adverse effects from painting in such low temperatures. The key is to keep exposure to the cold to a minimum and work quickly. However no advice can overcome the strange look you'll get from your partner when you take your little metal men out to play in the snow.