When the crown is gone beyond th point of simple repairs use Crown Saver to build a new one. Comes in 30lb. buckets with directions on the side. This is a dry product which you mix with sand and water. Can be used with or without Form Foil, an aluminum foil backed adhesive that is pressed in place over the existing crown.
You may apply Crown Saver over the old crown, no forms are needed and it sets up in under two hours.