Name: Bob
Message: How do I, and what do I use to dye auto seat belts?

You probably should not dye seatbelts that are intended to be used. If they have faded in the sun, the fibers may have been damaged by the sun as well, so it is best to replace them with new seat belts, which can better be trusted.

Are they made of nylon? If so, you can use acid dyes. The best black for nylon would be Lanaset black. 

All-purpose dyes contain a mixture of acid dye with a rather poor cotton dye, so they can be used in a pinch, but you must include vinegar in the dyepot, and heat to something like 185°F. for half an hour, to help the dye adhere to the nylon. 

If the seatbelts are made of polyester, they will require extensive boiling with disperse dye. No other dye type will work on polyester. You cannot dye polyester with all-purpose dyes, acid dyes, or reactive dyes.

You will need to buy a large non-aluminum cooking pot for your boiling dyebaths. Do not reuse a dye pot for food preparation, unless you stick to the use of food dyes only.  (Food dyes can actually be used as acid dyes on nylon, but they are not the most lightfast of dyes; that is, they will fade in the sunlight.)

If you do not wish to boil your seatbelts in dye (I worry about the possibility of weakening the fiber), you will have to use fabric paint instead of dye. Fabric paints will work on synthetics as well as on natural fibers, but they may not stick as well to synthetics, and they will wear off more quickly than dye. They may be more resistant to fading in sunlight, however.

Your seat belts are probably made of polyester. According to the book Handbook of Technical Textiles, by A. R. Horrocks and S.C. Anand, published in 2000, some early seat belts were made of nylon, but most seat belts today are made from polyester, due to its greater resistance to UV degradation. They wrote that seat belts are mainly black in Europe, and light grey in the US and Japan, but that this is changing in order to coordinate better with the colors of the interiors of cars.

Jacquard Products makes a fabric paint, Dye-Na-Flow, which acts like a dye but works on both polyester and natural fibers. The brief heat-setting with an iron seems less likely to damage fibers than extensive boiling, but strength testing should be performed to confirm this.

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[Updated Janurary 21, 2008]