Rit dye works best on natural fabrics, so polyester, which is synthetic, does not take the color well. The dye does not attach itself properly to polyester. When polyester is dyed with Rit, the color is likely to be uneven, and the dye sometimes rubs off on other materials or bleeds when it becomes wet. To color polyester fabric, manufacturers dye the fibers before they are woven.
Polyester fabric is produced from petroleum products. It is also coated with a finish that makes it resistant to wrinkling. As a result of this process, polyester has a moisture barrier that stops Rit dye from soaking in properly. However, rayon and nylon are synthetic fabrics that do accept Rit dyes.
Other materials that do not dye properly with Rit include acrylic, acetate, fiberglass, spandex and metallic fibers. Fabrics must be able to be laundered in hot water, and they should not require dry cleaning. Anything with a special finish, such as a water repellent, does not work with Rit.
However, Rit does dye polyester blends that are at least 60 percent natural fibers, such as cotton. The final color is even, but it does not turn a deep, rich hue. Natural materials that work well with Rit are silk, wool, ramie, wood, wicker, paper, feathers and cork.