I have recently painted a wood shelf that runs along the length of three walls in my finished basement. It is painted white, and I plan to use it to show off framed pictures and various accessories that I will use to decorate the room. I want to put a protective coating over the paint and would like to maintain a flat finish if possible. I began using a satin spray polyurethane, but it looks as though it may be slightly turning the white a little yellowish. Just wondering if polyurethane is the correct protectant to use on the shelf in this instance. Again, I would love to maintain the flat look, but it isn't neccessarily as important as getting the right finish coat, just in case an occasional can of soda or bottle of beer were to be placed on it.