However, that won't keep the paint from evaporating. If there's any air in the jar, it'll cause the paint to dry up. The only way to reduce the amount that's evaporated is to fill the container as full to the top as possible with paint. IOW, don't leave any space in the container for air. Obviously, that's not always possible. If you can figure a way to vacuum seal the container, then you might slow the evaporation rate. Once paint is opened, it's going to evaporate.