How to protect the wood finish of speakers.

One thing an owner needs to consider is the type of finish on wood. Most speakers of modern manufacture are going to be finished in some sort of catalyzed impermeable finish. No amount of beeswax, oil, or whatever is going to have an iotas effect on the wood. It may make the surface look pretty, but it's not going to affect the actual underlying wood unless you have an oil or wax finished veneer. And those are fairly rare these days. Conservators wouldn't trust sunscreen products to protect any valuable wood item. They may help a little, but don't be lulled into a false sense of security and think they'll stop sun bleaching, especially in places like Florida, Arizona, or Hawaii.

curtains on windows work pretty good

Photon46 is correct. Two options, curtains, or a UV filter film (made by 3M) installed on your windows. Also, periodically, switch your left with your right speaker around to get an even tan.

Try to protect the grills too.Also the sun may shine through the grill,damaging the drivers.

I just had my west facing windows covered with window tinting. It will achieve 3 major improvements to a room. First 98% of the uv light will be blocked-the most critical portion of light that causes fading. Second it will block 50-85% of the infrared wave length( infrared accounts for 50% of light's thermal energy). Third it will improve the accoustic's of the glass which is often a source of reflected sound perceived as bright,and fatiguing. 3-M is a good brand but there is a German brand Huber that is the best if you want your window protected but no tinting. This brand is used by museums that want maximum light but blockade of the uv and infared. It uses 3 layers of plastic each tuned to a different wavelength. It has no metallic sheen to it like 3-M. It is the best.
David Pritchard