I hung the rock in our house and had several arches and barrels....same issue. I sent this sketch to a guy on another site (forgot which one). The feedback I got was basically "don't drag the knife tight along the arch bead, leave a small gap so you get a small thickness of mud covering the notches, then take some extra care with the sanding". I still haven't finished all my arches.
Yeah, but the pic looks like normal mud "shrinkage" where the cuts are in the bead, not the knife hit the notches. The technique described is correct. And use a knife that is wide enough (8") to make sure you span the notches. Hand sanding with a sponge will take care of the rest.......
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