There are a couple of things that I use before a skull leaves my shop. 1st thing I do is water down Elmers glue, so it can spray from a spray bottle. This helps to hold teeth and make some of the more fragile bones more solid. It dries clear, so no worries with it yellowing.
For the actual sealing of the skull, I use product called Acryloid (Paraloid) B-72. It used by museums and such. I use this because I do into the 100's of skulls a year. I have folks mail skulls in from all over the US for me to do for them. If you are only doing for yourself, I would suggest a matte krylon non-yellowing sealer. Easily purchased at Michaels or Hobby Lobby.