When I purchased my Art Alternatives Gesso it did not have a bad odor, but now it does. Why is this?

The smell is most likely coming from a contaminant, which can come from many sources. Bacteria is present everywhere. If you dip a paint brush into the gesso (even a clean one!) you could unwittingly introduce bacteria into the mixture. A strong odor is a symptom of bacteria growth in your gesso. It is best to always pour the amount of gesso you need into a separate container and always dispose of the leftover gesso (do not put it back in the original container).

Note: bacteria grow much easier and quicker in a warm location so always store your gesso in a cool dry area.