Thanks my friend @tephen-w-towle! It is believed that a coat of clear varnish, or, even better, liquid decal film, such as the product from Microscale, might help. However, honestly, I had no luck in the past with the above methods, which, I believe, work more in a preventive manner, meaning coating the decal sheet BEFORE it is compromised by aging. Anyway, you have nothing to lose by trying (I would not apply Future/Pledge onto the decal sheet, since it contains water and might affect it).