I did what others suggested: Lay a 2" strip of masking tape around the perimeter of the football. Then put the glass in place and add the epoxy. Let the glass and epoxy slightly overlap the edge of the masking tape. When the epoxy is stiff but not rock-hard, use a sharp utility knife to cut along the edge of the masking tape. This gives you a nice clean edge that you can easily feather while the epoxy is still semi soft.

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