I couldn't find the right sort of fabric for the Monk's bodice and shorts, so I decided to buy a grey knit and paint the checkered design on. Last week, I did an experiment with fabric paint and acrylic paint. The reason was I didn't like the fabric paint selection in the store. They didn't have the right shade of red (it was more of a pink really), but when I went to the acrylics section, there were so many shades to choose from!
I started out by blocking out parts of my fabric with masking tape (using it for actual masking of things for once). Then I dabbed the paint on with a sponge. Be careful not to use too much side to side action when applying the paint because it might get under the tape. Once the paint is dry, remove the tape.
As you can see, plain acrylic paint on a knit fabric does not work too well. It doesn't stretch with the fabric, it's too stiff, and little bits of paint flake off. The solution? Fabric paint medium! This stuff is pretty awesome. You just mix it with the acrylic paint of your choice and yay instant fabric paint! It improves how much the paint adheres to fabric, flexibility, and how much it stretches. Additionally, it doesn't appear to alter the color at all. The only downside is the fabric medium tends to water down the acrylic a bit, so you might need to apply more than one coat. So yeah, next time you can't find fabric paint in the color you want, just make your own :-)