I generally sand before filling. If I had gaps large enough for a sliver (which I never do) I'd put in the sliver and/or do all my repairing, fixing, etc. before sanding. I find it easier, better, with a better all around job using straight cut strips rather than B&C...but that is a moot point for you at this point. After sanding, I coat the wood, search and fill any cracks with an epoxy/wood mixture, and put on the fiberglass. A word about sanding: I sand with a 60 grit ROS...that's it. Unless I'm gonna build a museum piece, or a boat that I'm going to show off I see no need in going further. One must determine what the intended use of the craft is and do the finish that is necessary. I believe there is a lot of agonizing about finishes that don't need to happen. Same thing goes for the fair surface. Do the best you can...then go on. You're gonna use the boat...aren't you...not put it behind glass in the living room? Of course, if it's really bad you can cut it out and do that small section over. Nobody has ever built a perfect boat. You will get better at it, though....and this first one will still be very impressive to EVERYBODY. Now...having said all that...there are times one needs to go ahead and do the museum quality work...but it's not on your first boat that you are building for personal use. Do the best you can....be happy.