Just use a clear varnish SA. It won't colour the MDF but it will darken it a few shades. Put about 3 or more coats on though and make sure you seal all edges etc as MDF really absorbs the moisture and will blow out quite quickly if it is not completely sealed on all surfaces. I have used Wattyl, White Knight and Estapol brands, (not all on enclosures though), and found them all to be good.