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Screen printing with white ink can be tricky and frustrating. The consistency of Speedball's white ink is a little different than other colors. With dark color inks, the ink soaks into the fibers of a shirt and stains the color into the shirt. Because of this, you can't feel the ink. White is a little different. It takes a thicker coat to cover the color of the shirt and get a bright white.
If you are going for a weathered, worn look, you will want to use regular white Speedball fabric ink. I try to use less pressure on the squeegee allowing more ink to pass through the screen. As that thicker layer of ink dries, it will begin to crack. After a few washes, you'll notice even more weathering. The photo below is also using Speedball white fabric ink. Because the lettering is skinnier, you don't notice the cracking as much. You can still use regular white ink on dark shirts and pull off minimal cracking by using thinner coats of ink and strategically choosing designs without large areas of solid white.
If you want a more clean, solid white, Speedball has an opaque line of fabric inks. The opaque inks have a shimmery quality to them. The coverage is closer to the coverage you get with color inks. To avoid cracking, you don't want to use thick coats. I have noticed that one thin coat of white opaque fabric ink still shows some of the shirt. In this tutorial video, I show how I use a heat gun to quickly dry the first coat of ink so that you can apply a second coat on top using the same screen. Using 2 light to average coats of ink, I am able to get a nice, solid white ink that holds up great wash after wash!
Supplies used in this video: