Paint Storage Tip
I have found when storing small amounts of left over paint in a gallon can that the air gap between the surface of the paint and the lid, (or bottom of the can, even if you store it upside down), can be enough to allow latex paints to skin over.
Also, over time, the paint will spoil quicker.
To avoid this, take a piece of lightweight plastic and apply it directly over the surface of the paint and an inch or two up the side of the can.
Don't use a plastic grocery bag with coloured printing on it as some inks used can be activated by the moisture and leech into the paint.
This will greatly increase the storage life of latex paints.