Once you’re happy with the shape of the board, it’s time to seal it. If you’re shaping EPS (Expanded Polystyrene) this will fill in the gaps in the foam. EPS is an open cell foam. It’s usually made of tiny foam beads pressed together. So just like a glass full of marbles, water can soak into it. Sealing it will prevent the resin from seeping into the pores while you are glassing. This means less resin used and a lighter board. It also means less chance the board might dry out your glass which is something I’ve heard can happen. I’ve also heard folks say that it will help keep your board dry if you get a ding but I’m less sure about that one.
The X6 is made from XPS (eXtruded Polystyrene) foam. Basically a chemical reaction causes the material to foam up and get pooped out a machine. This is a closed cell foam which is water tight. So what we’re doing in this step is to fill in all those nicks, gashes, and other boo-boos we got while shaping so we have a nice smooth board.
Different folks have different preferences for filler but I’ve seen these break down into two general categories. One is resin mixed with micro-balloons. The other is some sort of spackle. Because resin is expensive and less forgiving to work with, I go the spackle route. I use DAP Fast and Final which is really lightweight, mixes well with water and dries to a very nice white color.
First we tape the stringer. Then we go over the deep gouges with spackle right out of the tub.
Once we’ve got the big gouges filled, we mix some spackle with distilled water until it’s the consistency of yogurt, or mayonnaise, or whatever your favorite creamy food stuff may be. We’ll coat the rest of the board with this to fill all the smaller nicks, scrapes and gouges. On an EPS board this would be filling in the pores.
Mmmmmm not yogurt 
Filling in all the little nicks and scrapes
Coat one side, let it dry and then sand it smooth. I find 100 grit sandpaper to work well. We’re trying to fill in the gaps not create a shell on the board so ideally you should be sanding until you see little sparkles from the foam peaking out. Just don’t over do it.
After going through the process, you may find areas you missed or areas that need some additional reconstruction. Repeat as necessary to get the final form of the board.
