How can I redo my damaged window sills?

The window sills in my kitchen, dining room and bedroom have been badly damaged from our lab jumping on them to look outside. They are approx 4" in depth and made out of pine. Being a soft wood his nails have deeply scratched them so sanding down is not possible. Our windows are large and not very far up from the floor. We live in a country setting so there is lots for him to be curious about. Keeping him off may not be realistic.
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