A fantastic coating and preservation  for shells I have used is acrylic gloss
medium by the company that makes gesso ( the acrylic titanium white paint used
for priming canvases)  You can find it at most art and craft supplies stores
like MJ Designs or Franks. I use it after I prep the shell with  mineral oil (
or microscope imersion oil whichever I have on hand  and yes the immersion oil
works great)  Let the oil dry and then paint a watered down solution of the
gloss medium.
 
I have only used this method on 'Fresh" shells, I haven't used it on fossils.
Try it out on a few specimans that are broken or arn't your best before trying
it on your prizes.
 
The last time I was at calvert cliffs I was in the 4th grade on a field trip.
I don't even remember where we went. Where did you go?
 
Sarah Watson
Silver Spring MD
http://www.geocities.com/southbeach/tidepool/8845