As gjek said: Cold temperatures have a profound effect on coating performance. Acrylics are particularly vulnerable to low temperatures, although enamels are by no means resistant. If your garage is unheated, as I assume from your post, keep the paint and model inside until the last minute, paint quickly, and return the model to inside temperatures.
A radiant space heater (the completely enclosed, electric powered oil reservoir types that are not potential ignition sources) would be a good investment for the garage.