Originally Posted by tom43004
If you sand the cores flush, cover the parting plane with tape or something else thin so it protects from scratching and leaves the foam a couple thousandths proud.
I used steel feeler gauge stock for this operation. Its very hard (I think high carbon steel) nominal 1/2" wide. You can get it down to low-thou thicknesses in lengths or even rolls from machining supply places. Just light 3M spray tack it on the flange so it stays put, do the sanding, then pop it off or use a little alcohol on the edge so it doesnt bend or kink. With steel you wont ever have to worry about sanding through it & scuffing up the mold flange.