This is what I use for keeping acrylic paint wet and usable for longer periods of time. I wet a sponge with water, wrap it with wax paper. I put this in a Tupperware that will fit it with paint on top. I put on top of the wax paper my colors of acrylic paint I want. I use it like a pallet and mix my paint with a drop or two of Liquitex slow-dri blending medium. When I want to save the paint for later I close up the Tupperware. I’ve had this keep acrylic paint for up to a month of use. I use the Krylon crystal clear acrylic coating when my painting is finished and dry.