If by "mix a quantity", you mean add thinner and store, I'd advise against it, as most paints don't keep well once reduced or thinned. If you're just decanting, what I've done to seal up glass paint jars more effectively is to cut snug fitting discs of soft plastic to tuck up inside the lids. I keep the tops from coffee, oats, etc. for that purpose. They provide a nice friction free seal, and paint doesn't stick to them and bind.