Which sealer to use for custom made background?

A custom made polystyrene/quikrete background, which sealer would be better, as in, lasts longer, is less toxic, and looks less glossy...

http://ift.tt/28WLxTW

http://ift.tt/28WLkjE

http://ift.tt/295BhFX

http://ift.tt/295BfOl



https://www.youtube.com/watch?v=ROZPt7M2uIk

Or, can you suggest a better one?