The centerfin box is installed after the hotcoat, before sanding the board. The lip of the box will be sanded down flush with the bottom of the board during the sanding process. 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.
Grind the finbox plastic down for a few seconds and let it cool for a few seconds as too much friction will actually melt the plastic and the heat can possibly transfer into the foam and weaken the area around the box. So it's best to cycle the grinding a few seconds on, a few seconds off.
Grind the box down as close as you're comfortable with without hitting the hotcoat. Then proceeed to sand the entire board with the flexpad flat and as you pass over the finbox it will sand evenly with the hotcoat.