I’ve wondered how much truth there is in the old rule that you shouldn’t paint if it’s raining?
Especially, in a modern house with Air Conditioning. Will moisture outside really harm the painting inside? Will it help if we keep the doors & windows closed?
It seems like anytime a big painting day is planned, the weather brings rain. In the real world it seems unlikely contractors and painters lose valuable inside work time just because it’s raining. Or maybe they do?
It seems like this rule made more sense in the days when houses were cooled with open windows and fans.
From my understanding, when it rains the humidity in the air goes up therefore increasing the amount of time needed for the paint to dry. The more time it takes to dry the more likely dust and other things can stick to the paint. The same works with staining and finishing wood. So, the dryer the air the faster paint dries. Even though the inside of the house is air conditioned the humidity does go up inside, but I wouldn’t think it would be anything that should stop you from painting a room.
Unless you’re living in an extremely humid area, rain shouldn’t affect your indoor painting. Where I live, for example, the environment is borderline arid/desert conditions, so even a couple days of solid rain won’t typically affect the condition of the paint.
Any airflow, even humid air, will usually help to dry things out more rapidly, so running your A/C or furnace will also help. Likewise, if you have a dehumidifier that will also help. But the technology used in the more modern latex paints sets the product up to touch-dry in an hour or less, typically (in some cases much faster than that even), so I wouldn’t worry too much about it.
I would think this to be an issue only for water-based paints. Latex house paints are typically water-based, so this will matter for them. For stains/varnishes/finishes that use mineral spirits, this shouldn’t be an issue. Likewise with cans of enamal/lacquer spraypaint.