Errr... Yes, we do want the fins to stay on. Admittedly, the previously mentioned cardboard and duct tape method has sometimes failed at this.

It may be a crime against humanity and rocketry to even mention this, but we've found that hot poly glue even works, as long as your casing stays cool enough. Unfortunately, we're using poly casings right now which reach about 250-275 degrees (F) during the burn, so hot glue does what hot glue should at that temperature. When the shell stays cool though, the poly glue has withstood direct fin to tree branch collisions at high velocity without giving way.

When we want to waterproof wood surfaces, we typically just use polyurethane, but I suppose the epoxy would be faster to do. Interesting.