Name: Grace

Message: Hi, I want to dye my swimsuit. It's 80% polyester and 20% spandex. Would disperse dye work? Thank you! 

No, you cannot use disperse dye on a swimsuit that contains any spandex at all. Spandex is very heat sensitive. In general, the laundry care instructions tell you to never use water about 105°F (that's 40°C) to wash spandex-containing fabric. This is because the spandex fibers are heat-set. Subjecting them to high heat may cause the shape of your garment to be ruined.

Disperse dyes require high heat to transfer. You must either boil the fabric with the dye for an hour, or apply the dye to paper and then transfer it with a hot iron. In either case, the heat is high enough to almost certainly ruin the swimsuit.

Other types of dye will not work at all. Dyes that work on natural fibers will not stick to polyester. Disperse dyes are the only type of dye that will work at all on polyester, but they cannot be used at temperatures that are low enough to prevent damage to the spandex. This means that polyester/spandex blends are undyeable, in every case.

There is an alternative. You can use an acrylic-based fabric paint, such as Dye-Na-Flow, combined with the additive Jacquard Airfix before use so you don't have to heat-set it. Some fabric paints, such as SetaColor, are reported to become permanent without heat setting or catalyst if they are allowed to dry for a full month. Fabric paint will wear off much more quickly than dye, since it does not penetrate the fibers as deeply, but it is your only alternative for coloring a polyester/spandex blend fabric. It's also more resistant, in many cases, to fading from direct sunlight.

Another option you should consider is buying a new white cotton/spandex swimsuit, which would be very easy to dye in any way you choose with cool water fiber reactive dyes, such as Procion MX dyes. Dharma Trading Company sells a variety of one- and two-piece cotton/spandex bathing suits for very reasonable prices by mail order.

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