It is possible to dye polyester, but not by using Rit or any other dye that works on cotton. See "Dyeing Polyester with Disperse Dyes".

An especially convenient way to dye a cotton/polyester blend is to buy some Jacquard iDye, which is a direct dye that will color cotton, with Jacquard iDye Poly, which is a disperse dye that works on polyester. They can be applied at the same time, if you have a sufficiently large dyeing pot to boil the sweater on the stovetop with the dyes. Obviously, this assumes that your cotton sweater has been fully preshrunk already, as otherwise shrinkage will make this impractical.

Rit All Purpose Dye is a particularly poorly washfast dye—that is, it fades quickly when washed, and tends to ruin anything else in the laundry you wash it with—but so is iDye. This is because both contain direct dye, which is inherently poor at resisting washing. If you choose to use either of these dyes, I strongly recommend that you mail-order a commercial dye fixative, such as Retayne or iDye Fixative. This set the dye, although both of the remedies suggested by tradition, salt and vinegar in the wash, will fail. The commercial fixatives really do work, when applied as directed. I consider both all-purpose dye and direct dye to be unacceptable for most uses unless they are set afterwards with one of these cationic dye fixatives.

Alternatively, you can apply the polyester dye and the cotton dye in separate steps, and substitute a really good cotton dye, such as Procion MX dye, for the direct dye. You can't apply fiber reactive dyes such as Procion at the same time as the polyester dye, but they are far more permanent and satisfactory than all-purpose or direct dyes, and they do not require a cationic dye fixative, only soda ash or washing soda. Unfortunately, going through two separate dyeing steps is more trouble.

However, I must warn you that dyeing will often fail to cover up a stain. All dyes are transparent, so the stained portion of the sweater will continue to be darker even after you dye it. In addition, some stains will themselves resist dye. Oily stains are the worst, in this respect. Do everything you can to clean the sweater before you ever try dyeing. Wash in hot water with Pinesol to remove oily stains; use Rit Color Remover to remove dye stains; use a good stain remover and/or soak in color-safe oxygen bleach, one such as OxyBoost that contains perborate or percarbonate. Whenever you redye clothing, it is important to get it as clean as possible before you dye it. Avoid using household bleach, however, because the hypochlorite it contains will permanently damage synthetic fibers.

If you want to buy Jacquard iDye and mix it with Jacquard iDye Poly, you will almost certainly have to buy the dyes by mail-order. You can buy both of these dyes from an art supplier such as Blick Art Materials or a dye supplier such as Dharma Trading Company. For the two-stage method, using disperse dyes as well as fiber reactive dyes, you can buy both superior disperse dyes and fiber reactive dyes from PRO Chemical & Dye, in Massachusetts, or from Aljo Mfg. in New York. See "Sources for Dyeing Supplies Around the World".

(Please help support this web site. Thank you.)