Hi Dean,

Just sand them out and recoat. The polyurethane is there for UV protection only (the shine is a bonus). You can't expect it to withstand a run in with a rock. My Arluk III with a Gelcoat bottom has the same scratches as you describe with the poly and varnish. I think that you will need something harder than the rocks you are hitting to stop the scratching. DIAMONDS perhaps!! {:o)

You will find that the polyurethane will stand up to the UV far better than varnish.

Ian