After many hours of research, my sand dollar dilemma is solved! I tried Krylon, modge podge, a clear gloss seal (used on my kitchen table) and clear fingernail polish. Nothing worked. Well, now something does!
If you ever want to write on sand dollars, here's what you do:
Two coats of Pledge Floor Care Finish (with Future Shine) -- apply with a cotton swab -- wait at least 20 minutes to dry and use a Sharpie paint pen to write. The protective wax coating (a) makes it easier to write (no longer a chalky surface), (b) saves the paint pen from drying up faster and (c) prevents the paint pen from bleeding (as it would on the original porous surface).
Questions? I'd love to help!
http://www.thejannafont.com