If the Master is situated in an outdoor space, we would recommend that you render it in dry conditions. We would suggest a temperature above 41°F.
Any lower than this and the render will take a very long time to cure; not only that, the water in render could freeze, significantly weakening it.
We would also not recommend rendering a Master Oven in temperatures over 86°F and in direct sunlight, as the render can cure too quickly, making it unworkable.