I need to repair a deep crack on my corian countertop. Do I need to fill the cracks first and then sand? What kind of compound would be best to use and what kind of sandpaper? Thanks

If the cracks are less than 3 sheets of paper wide, they are quite easy to fix. Go to Discover Corian® (art specialties international, inc.)

There is a page there for ease crack repair for Corian.