After cleaning and peroxide or bleaching a skull mount, do taxidermists put any kind of sealant on it?

I`m not a taxidermist, but do the european mounting myself. I have never sealed them in any way, and I don`t think it`s necessary.. If cleaning/bleaching is done properly they will stay white. If there`s any fat/grease left, it will turn out as yellow spot after a while.. Hope this helps you a bit.

Yes , I seal mine with elmers and water mixed 70/30. I make up a batch in a tub so I can dunk the skull then set it out to dry.