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. Paint products can become contaminated with bacteria or mold, which can lead to a strong, offensive and in some cases, sickening odor. Paints such as poster paints and temperas that are intended for classroom use, and for children, often contain an organic binder that is subject to degradation if not stored properly, or used within a reasonable period of time. To guard against bacteria and mold, manufacturers of paint products add preservatives.