The addition of water dilutes the preservative's strength as well as the paint. If diluting paint to simulate watercolor techniques, prepare only enough for immediate use. Avoid working directly from the original product container. Do not place brushes, hands, or other objects in the container. After each use, make sure the cap is returned tightly and that the product is sealed before storing.
All paints are subject to eventual spoilage once opened and exposed to air and other contaminants.
Most spoilage is a result of cross-contamination from common sources such as air, water, people, brushes, and other utensils. Proper storage and usage will reduce potential sources of contamination and extend the life of your paint.