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Keep your screen tight. Your screen should be stretched extremely tight across your frame. After using a new screen for a while, it will be come loose. This Screen Tool comes in super handy to push the vinyl spline back into the frame and tighten the screen.
Apply a good amount of pressure to the screen and squeegee as you move the paint across the screen. You really want to press down hard to create a seal between the screen and your shirt. Otherwise, your paint may bleed. If you are noticing your design lines aren't sharp and crisp, try pressing down a little harder on the screen and squeegee.
The type and brand of transfer tape makes a difference. If you have trouble with your design not coming off the transfer tape and sticking to the screen, you might want to try a transfer tape that isn't quite a sticky. My favorite transfer tape is one I sell in my shop: Transfer Tape
Scoop up your extra paint on your screen and squeegee when you are done and put it back in your paint jar. No use in wasting perfectly good paint!
Experiment with different colors beyond the basic colors that Speedball offers. Did you know that you can mix your screen printing ink? I offer a bundle of Ink Recipe Guides to help you learn how to mix new colors using some basic Speedball fabric inks.
Wash and try again. If you make a mistake on your shirt, sometimes I've been able to immediately wash the shirt in water and Dawn dish soap to get all of the ink off. I have found this works with dark shirts and light color ink. I have tried washing off black ink on a yellow shirt and it didn't work.
Touch up with a paint brush. If you design is perfect except for one spot that didn't print well, I have used a fine tip paint brush to touch up. I haven't had any luck lining a screen back up after it has been removed, so I do minor touch ups with a paint brush.
Use the same screen/design multiple times. Screen printing really becomes more economical compared to heat transfer vinyl (HTV) when you are making multiple shirts with the same design. Plus it is so much faster, just cut one vinyl design and you can re-use it over and over. The catch is, you need to reuse it immediately, before the paint dries on your screen. I have lined up 12 shirts and done them all with the same screen! After screen printing one shirt, just lift the screen off and place it on the next shirt. Add more paint, squeegee and repeat!