I had a local glass company make the piece of glass which is only 1/4 inch thick. They said that it didn't need to be too thick since it just sits on the top of the furniture. My glass was about $100 but it is extremely long at 79 inches. My local Home Depot said that I should get steel wool to scuff up the underside before spray painting. The steel wool didn't seem to do much except clean the glass, which was helpful but not necessarily needed. I have included a picture of the steel wool and white spray paint that I used. Then I simply spray painted one side after taping off the edge of the glass.
I cut a piece of old wrapping paper and placed it on the top of the furniture with the white side facing up. I was worried about the paint transferring to the credenza top and it makes the white just a little stronger in color. The most difficult part of this project was transporting the glass, which I did by laying down a comforter and pushing down the backseats. The guy at the glass company insisted that it wouldn't fit, but luckily I know how to work the tape measure:) He was actually very nice and even said that he shouldn't have second guessed a woman which was quite funny. Now, on to decorating the tabletop. As always, feel free to comment or ask questions!