Name: Karl
Message: Is there a chemical that can stop uv sunlight from fading dyed fabrics?

Maybe yes, maybe no, depending on the specific dye with which you are concerned. There are spray-on ultraviolet protectant coatings, but you must TEST to see whether they actually protect the dyes you are interested in. In testing, one of this type of coatings actually accelerated fading of one fiber reactive dye! The problem is that a product that absorbs ultraviolet energy might actually pass this energy on to a certain dye, thus degrading it even faster than usual. See the text on the page "Lightfastness of Different Types of Dyes".

There's a possibility that the best ultraviolet protection for dyes in clothing might be provided by a Ciba product called Tinosorb. Unlike the popular spray-on product called Quiltgard, Tinosorb actually stays in fabric for a number of launderings (at least twenty). I have purchased and used this product in the form of Rit Sun Guard dye. This colorless dye is sold for the purpose of increasing the SPF value of cotton clothing; unfortunately I have not tested to determine whether or not it also protects the other dyes that have been used on the same fabric. It does not, in my experience, change the color of even white fabric, and it does help prevent sunburn. It might be worthwhile to test whether or not this product will also help to protect the dyes in your fabrics. [Addendum: Karl tried it, and it did not help.]

Note that it is not only the ultraviolet light in sunlight that fades dyed fabrics. The same visible light that makes it possible for us to see the color of the dyes also causes fading. Compounds such as dyes are colored because they absorb part of the visible spectrum, but the energy of this absorbed light can serve to break chemical bonds. Dyes vary in their resistance to this fading, and some will be affected more my UV light, while others are affected more by visible light.