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Oh, you can use a LOT of stuff to coat crowns with varying degrees of effectiveness. There are a number or products available through chimney sweep supply houses that all shed water. The problem is, almost all are NOT vapor permeable. Most I have used destroyed the crown in short order. They trap moisture under the membrane causing it to fail rapidly. For simple repairs, I currently use a high lime stucco mix followed whenever possible by water repellent treatment using ChimneySaver. To me the best would be a true lime mortar with no Portland cement in it so it can breathe better. Regardless of what you have available, look up the specs on its vapor permeability and go for a high perm rate.
A corollary would be never to paint masonry including stucco. I've see even highly rated "high permeability" stucco paints fail within one year. The BIA recommends no coatings unless 100% vapor permeable.
HTH,