Though white is traditional, using other colors for your underbases can help you create incredible designs on dark garments.
Blocking out the shirt color and matting down its fibers is an underbase’s most important job. When using plastisol inks, print the underbase with light squeegee pressure so that the ink isn’t pushed into the garment. The higher the underbase “sits” on the surface of the shirt, the brighter it will look and the more shirt fibers it will cover, benefits that are lost when squeegee pressure is excessive. Some printers try to counter this problem by double printing (and double flashing) the underbase in an attempt to better seal off the background. (I call this approach “revolvering” because it forces you to index the press in the wrong direction.) This just isn’t a profitable way to produce T-shirts, so avoid that technique whenever possible. With the right screen and minimal squeegee pressure, you shouldn’t have to resort to double printing the underbase.
Regarding reflectivity, the simplest way to envision this is to run a colorful photo through an inkjet printer on bright, white paper, and then print it again using a gray or black stock. You will quickly notice that the darker paper makes the top colors appear much duller. (On a black paper, they may not even be visible.) The reason images appear brighter on a white underbase is that almost all inks are somewhat transparent. In the case of deep, saturated colors like bright red, royal blue, and lemon yellow, the pigments in these inks are like stained glass windows. They
create these bright colors by capturing every other color except the one they let through. So a royal blue pigment traps every color wavelength except bright blue, which is what our eyes see.
However, the degree of brightness will vary depending on how much light bounces off the surface underneath the ink. A darker paper (or underbase) will reflect less light, so the blue will seem much duller. The whiter the underbase, the more light that is reflected and the brighter the colors will be. (I should note that some inks have different levels of opacity and pigment saturations that can affect the recommended procedures for printing underbases. With the majority of semi-transparent inks, a white underbase will provide the brightest overprinted colors.)
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