The best and most common form of protection from fading would be the selection of UV glass or acrylic. Any framing store will have these options available at multiple price points with trade offs such as glare or less glare, scratch resistant or scratch prone, and tint(green) or no tint.
Specifically you are asking for a form of UV protection that is spray on, and this type of protection does exist, I would try Krylon UV Resistant Clear Coating Spray.
This product is made for use on artwork that is either outside or near windows. They offer the product in gloss and matte finish as well. Krylon is a very well respected brand, and I believe this will be the easiest and cheapest solution to the problem, albeit not necessarily the best.
Another spray on option would be Premier Imaging Products Print Shield. It is a lacquer based protective spray that is designed for inkjet prints. It also works on photo papers and canvas materials. They specify up to a 2x lightfastness increase, elimination of gloss differential, non yellowing formula, and a reduction in scuffing.
The results have been tested by an independent lab (Wilhelm Research) to last up to 166 years with certain Epson papers and regular glass, and over 200 years with UV filtering glass.
As with any products like this I would suggest testing them out on a test print before you go and use them on your framed and mounted originals.
Above I have given you a few different options, to sum them up I will list the following options-
- UV Filtering Glass
- UV Filtering Acrylic
- Krylon UV Resistant Clear Coating Spray
- Premier Imaging Products Print Shield
- Picture Shields inserts (not described here, but link here)
To get down to the root of your issue, it sounds like you want to try to add UV protection without disassembling your professional framed and matted prints. Many professionally framed prints will be finished in the back, such that you have to open them up to do any type of glass exchange or sprays. I would not attempt to spray the exterior glass without first removing the glass. It just does not sound like a good idea to me, and you will probably end up with lines from the tape, or worse - over spray on the frame.
Furthermore, application of a spray type UV protection can be quite difficult to do in a uniform and smooth fashion. You may consider having the images professionally reproduced(scanned) prior to the application in case you are not satisfied after some time. You also might just be better off with the addition of UV filtering glass, keeping the image unaltered.
Beyond the scope of just using UV filtering vehicles, you will want to understand how prints get UV light, and other ways that prints are damaged over time.
UV light rays occur not only in natural light from the sun, but also in smaller quantities from indoor lighting sources. Florescent lighting is especially prone to outputting harmful UV rays, so if possible limit this type of lighting near the photos you are displaying. You will want to keep frames away from heat sources like fireplaces or central heating vents, because this type of added heat can cause premature aging of the photos as well.
It is important to note that UV protection will not eliminate fading, it will only limit the amount of harmful UV rays that will contact the photo. Other factors can contribute to fading, such as pollution, humidity, heat, and the paper that was chosen for the print and surrounding matting.
Another benefit to choosing UV filtering glass is that many reflections are often removed by this process, providing a better viewing experience.
Overall, I think you have quite a few options at your disposal. Realistically, in today's digital world, if you created the images, hold on to your RAW and/or high quality JPEG files and you can reprint if needed. If you didn't create the images or you do not have the originals, you have many options that are tested and work very well. Light is the print's enemy, so you could consider moving the prints to a darker room, getting better blinds or drapes, or even rotating them out seasonally to the closet.
For very detailed information you might find the ISO specification ISO 18902 useful.
A source I found, that reads mostly like an advertisement for a product, but seems to have useful information anyways: Here
I also found an interesting first hand account with some of the products listed above here.