Pretty much all modern wood finishes are impermeable, no oil or cleaner is going to penetrate any finish unless it's a rubbed oil finish. Basically you have two ways to stop sun bleaching, either cover the speakers when not in use or apply a UV barrier film to your windows. I've applied UV blocking film to all exterior windows in our home to protect our furniture, art, carpets, etc. and to reduce solar energy transfer into the home. It has really noticeably helped cool the home when the sun is shining and I no longer have to worry about sun bleach damage. The only negatives are that it was a time consuming DIY job and if you have rooms lacking light, the film's reduction in light transmission wouldn't be welcome.