Here is a tutorial for "hydrodipping", or swirl/marble painting tumblers with spray paint. I know it is not "REAL" hydrodipping, I am just using that term to reference the fact that I am using water as a medium of coating the cups with paint. Before people complain that spray paint is toxic and unsafe for cups, please do not bother. That information is incorrect according to the people at Rustoleum. Once the paint dries, the chemical that is "toxic", aka VOCs, are no longer present. This means that the paint is no longer toxic. Some people even spray paint their childrens beds, toys, cribs, etc. It is perfectly safe.

I seal my tumblers with a food safe epoxy clear coating.