There are different ways to smooth and/or strengthen balsa fins for model rockets while adding only minimal weight. The method illustrated here will do both at once. It will make the fins quite a bit stiffer/stronger and make them very smooth. You can use glue and printer paper to do the same thing. I used full-page label/sticker paper as I had some available. Using the self-adhesive label paper takes up very little time as there's no waiting for glue to dry. There's no mess to clean up. Balsa fins re-enforced in this way held up fine to an F40W on a TLP SAAB. It will almost certainly take bigger motors without trouble.

Remove some of the backing from the adhesive label paper and place the fins on it. The sticker paper shown here was purchased at Staples. Other office supply stores will most likely carry it as well.

Using a sharp hobby knife, trace/cut around the edge of the fins and remove them from the adhesive label paper. Do this for each fin. You have just laminated the first side of your fins.

Now place the bare side of the fins on the self-adhesive label paper. Trace/cut around the fins with a hobby knife (with sharp blade) again to remove the fins from the label paper.

Your fins are ready for use now. The next section is optional and will not improve the strength of the fins. It is done merely to smooth the remaining fin surfaces.

Set a bare side on the fin on the label paper. Once again trace/cut around the surface with a sharp hobby knife.

Repeat this for each side of the fin, except for the root fin surface. You want to ensure the fin adheres well to the body tube. Putting label paper on that side would make it so you are gluing label paper to the airframe, rather than the actual fin. In case you're wondering, the fins shown here were used on a TLP Gabriel (the aft fin set).