I’m pretty slow at building, and I tend to over do everything. Therefore, I decided to taper and fiberglass the fins. While it only adds about 5 grams of weight, it seals the fins, and the paint tends to coat them so much better. Besides, it adds a nice hard shell to fins to prevent minor nicks. The downside is that it takes time…
To start with, I rounded the leading edge and put a taper on the trailing edge. These are pretty thick plywood fins, so it took a while.
After sanding and cleaning the fins, I started laying up the fiberglass. I’m using 3/4 oz/yard woven fiberglass with West System epoxy. This is a hold-over from my RC airplane days. If done correctly, the fiberglass and epoxy adds only a small amount of weight. The trick is to drizzle only enough epoxy over the fiberglass to wet the weave, then squeegee any excess epoxy away. This will leave a slightly textured surface that will be removed by a subsequent coat. To squeegee the material away, I just use one of Jason’s old common Magic The Gathering cards (don’t tell him…).
I only applied fiberglass to the exposed surfaces. This process is pretty controversial as many people believe this adds weight with no real value as the fins are strong enough as they are. While true, I like the finish that I get and the paintability of the surface when done is amazing. I even do this to low power rockets. I compared the weight of fiberglass to sealing the fins with paper/CA, and the fiberglass came out slightly lower weight with a much better finish.
After curing, I’ll apply a second coat of epoxy (very thin, scraping most of the material off with a card), then a thin layer of glazing putty. I wet sand between all layers, careful not to sand too far into the weave.