Just a note: if you're using fiber paper and marshall's oils, you really don't need any kind of spray or varnish after you've finished the piece. Behind glass, any marks or 'floating' will disappear. I just don't like to put anything on my print that isn't absolutely essential.
FWIW, you don't need marshall's oils if you don't want to pay the price. You can use regular artists oils -- just have to be a bit more careful with them and thin them down so they're not so opaque.