The paint i keep sitting out (meant to use the next time i paint) is usually keep in the jar with the cap just laying on top to keep the bugs out. Usually get down to paint two or three times a week. If i put it up on the shelf and not sure when i will use it next, always make sure it is closed tight. My paint area (mind you i am in Ohio where there is plenty of flux in temp and humidity) is down in my basement and has some humidity year round but a fairly consistent temp of 55°. Never had an issue that i know of with leaving my powder out with the cap off. I lay the cap over top because i have learned my lesson that there are a ton of rolly polly bugs that end up in my jars if i leave them without any cover.