I've installed some un-backed curtains and have been told that after a few years they may start to disintegrate from being exposed to hundreds of hours of sunlight.
Does anyone know of a product, like UV sprays, that will help protect against this?
You can simply hang sacrificial curtains (muslin, perhaps) behind the curtains you wish to preserve. There's also the distinct possibility that this is pure sales-hype, as I've seen many an old curtain that looked just fine.
In any case, even if you block ALL UV light, that will not prevent fading, according to this site: http://www.fsec.ucf.edu/en/consumer/buildings/basics/windows/fading.htm Most relevant paragraph quoted below.
The most authoritative research on quantifying fading damage was done in the early 1950s by the United States National Bureau of Standards. The U.S. Library of Congress undertook this research, in order to design a glass filter to protect the original copies of the Declaration of Independence and the Constitution. Scientists found that blocking all of the ultraviolet radiation portion of the solar spectrum would not eliminate fade damage for most fabrics, but will slow down the rate of fading by a factor of about three.
I didn't see "fabric disintegrating" as a common complaint when looking into this.
Many companies sell "UV blocking window film".
3M is one such company and they claim that their film can reject "up to 99% of damaging UV rays that fade fabrics, window treatments, wallpaper, wood floors, photos, paintings and other valuable furnishings.
I'm not neccessarily recommending 3M film, it's just one of the vendors that came up in a Google Search for UV Blocking Window Film
There are UV blocking sprays for fabrics that might also be helpful, but I think the window film would be more effective (and wouldn't need to be reapplied after washing, etc)