I've run dozens of tests looking for a solution to this problem,
The above photo shows four timbers, clockwise from the top left are Oak, Sycamore, Black Walnut, and American Cherry. The left hand side of each board is unfinished, the right hand side is finished with a high quality yacht varnish that contains a high level of very expensive UV filters. The top half shows what a few months in a south facing window can do, the bottom half was masked off.
There are no silver bullet solutions (at least none that I've found), the best I can say is as follows,
-Pick the right timber for sunny environments like south facing rooms or conservatories. Most fruitwoods like Cherry hold up to sunlight well, as does Laburnum. These actually darken and patinate in moderate sunlight. The worst timber is probably Black Walnut which fades to a beige mud colour in next to no time. Brown Oak is actually pretty resistant to fading as timbers go!
-Morrells are working on a finish that offers some hope, but it's very expensive and spray only, plus it'll only delay fading rather than prevent it.
-Bog Oak doesn't fade, and neither do timbers that have been given a scorched finish.
-There are some relatively new stains called metallised acid stains, examples are TransTint and Colortone (available from the luthier supply company StuMac). I've tested TransTint quite extensively. It's genuinely excellent, far more light fast and fade resistant than any other stain. This will get you at least several years of fade resistance, but not several decades.
-Pigments can offer more fade resistance than stains, but of course they're essentially paint, so will obscure grain.
-One of the ironies is that UV filters are themselves broken down by sunlight, so they're fine for say yacht varnish that's intended to be scraped and replaced every few years, but they're not in themselves a permanent solution.
The Victorians thought sunshine was the enemy so drew their heavy velvet drapes. We want our modern homes to be washed in daylight, I guess we just have to get used to the inevitable consequence of bleached and faded surfaces.
