Varnish for acrylic painting on unglazed ceramic
Reader Question: I'm going to use acrylic paints on an unglazed ceramic plate. What varnish can I use?
If you are going to paint with acrylics on unglazed ceramic and you don't plan to fire the ceramic in a kiln, then after the acrylics have dried you can seal it with a water-based polyurethane, such as Jo Sonja's Water Based Polyurethane Varnish (this links to Blick Art Materials, and if you make a purchase I get a small commission that helps support this site). You should use several coats. You can also buy water-based polyurethane from a hardware store, but make sure to get something that says it dries clear and non-yellowing. (Sherwin Williams is a good brand to look for, but be sure to read the label carefully so that you are getting the right thing.) Since you are painting with acrylics, be sure to get the water-based stuff rather than the oil-based polyurethane.
This technique has been known to work quite well for sealing unglazed ceramics that have been painted with acrylics; however, it is not guaranteed to last a lifetime. A few people have reported that after several years, the acrylic paint flaked or rubbed off. If you want your ceramic plate to last a long time, you could keep an eye on it and re-varnish it every few years, if needed.
Hope that helps and best of luck with it!