The problem with trying to keep latex paint fresh isn't with the paint itself, but with the paint container. From wait I've noticed is the metal containers because of the water mixed in with the paint, will eventually oxidize the metal. Usually the rusting continues until the can is compromised and the paint solidifies. The best way to preserve a gallon of paint would be to add a plastic or rubber membrane so as to increase the seal between the can and lid. A garbage bag would do. Remove all paint (dried or not) from the lid well (groove). Place the new seal on the can followed by the lid. Gently tap the lid until completely seated. Invert the can to help increase the seal.