My other hobby is building model cars (automobiles) where impeccable glossy finish is a must. Ambient temperature and humidity are very important factors to be considered while trying to achieve perfect finish.
When spray-painting (either using spray can or an airbrush) when the compressed propellant leaves the nozzle it cools down considerably (
laymans explanation ). If the ambient air is humid (containing water vapor), that cooling effect will often be enough to condense the water right out of the ambient air into the cool paint stream. If you mix the water with organic based solvent like TruColor paint, the result will be not very pretty. Of curse the moister and warmer the air is, the more severe the problem will become.
Another problem can occur when the object being painted is cooled down by the cold paint hitting it. Again, if the temperature of the object is low enough for the moisture from the warm and moist ambient air to condense on the surface of the wet paint, you will get blushing - a foggy finish.
I live in Northeast USA where it gets quite warm and humid in the summer. If the dew point is in the 60s or 70s, I won't even go near my airbrush. I prefer spray-painting in the winder or on dry days during spring or fall. There are also dry days in the summer, but I usually err on the side of caution.
The above also applies to casting 2-part urethane resin. It is also very sensitive to moisture and if used on humid days, it can develop tiny gas bubbles in the hardened resin. That is why I also prefer casting in the winter months.
