I love the look of polished stones, but I'm not going to tumble my fossils!
I also have fossils that are constantly "shedding" bits of sand, gravel, and brachiopod parts.
Yes, I have the expensive, special order Paraloid/Acryloid B72 and the acetone to cut it with. But its a hassle to mix up.
I have been using ModPodge with great success! And it reverses with just a soak in acetone, just like B72. Inexpensive, available at any craft store, no mixing, comes in matte and gloss, but even the matte has a little shine. It is a glue, so it keeps them from shedding if sandy or fragile.
Also, if you rub mineral oil onto an agate and bake it for 20 minutes or so to help penetration, it comes out with a "poor man's shine".
Just an FYI. :-D