Greetings Ellen (and Happy St. Pat's Day too
from what I know Shellac might be the best sealant for collograph plates.
I have applied three to four coats of brush or spray can water-based varathane and this seems to work fairly good. I did try acrylic varnish and acrylic house paint but unfortunately this wasn't such a good choice as there was a tackiness to them and the damp blotted printmaking paper ended up gluing to the plate when it was passed through the press. Perhaps I didn't leave it long enough to cure?
I have also put plastic food wrapped over my plate and run it through the press with the blankets to mold it over the relief forms before applying ink. This works too but on a more temporary basis I think as the wrap can develop tears in it. Also you want to seal the sides and backside of the plate to make them totally waterproof for cleaning.