We recommend "grinding" the fin box plastic with 60 or 80 grit and a hard or medium flexpad. Grinding is a bit different than sanding whereas you tilt the sanding pad up a little bit to use just the edge of pad to focus the force right where you need it (on the box plastic). Sanding is done by keeping the pad flat on the board and removing a broader area of hotcoat. entire board with the flexpad flat and as you pass over the finbox it will sand evenly with the hotcoat.