Hi Martin;
I agree with the above members where a spray lacquer is an excellent choice and easy to apply. Brown paper bag. Thanks for the tip. Something I haven't tried.
Another process I've used being I prefer a natural finish is cabinet makers wax. I'll cut out three or four sizes of birch I need for the portrait or scene, router the edges to my liking, apply the wax and finish each board. Then I stack the pieces, apply the pattern and cut away. When done they are finished except for the one that had the pattern attached. Just an idea that works for me. Everyone has their own liking when it comes to finishing a project. Good luck and I hope your portraits turn out beautiful.