First off, don’t be misled by the title. I am only talking about how to get the form back on a fin that has scraped over rocky bottom.It is impossible to repair a fin that has snapped in half.
This also goes for fins that have been sailed over sandy ground a few times to many and have flattened the smooth curve to the tip. This also usually implies that the tip has been formed into a sharp point. These changes in the fin shape will greatly influence the efficiency of the fin and cause more spin outs than necessary.
Depending on how badly the fin has been damaged. If you have gone over rocks, it is very likely that the edge has split open and you can see all the individual layers of materials that make up the fin. If that is the case you have a lot of sanding to do if you use sandpaper. For the rough sanding it is best to use a rasp.
Then for the fine sanding you can use sand paper. There is a trick you can do though. If we just take the sandpaper in our hand and start scraping the edge, we will not get a good result because of the uneven surface and the small area of our fingers. What we have done at the surf center is to wrap some sandpaper around a block of wood and staple it on to make the sanding easier and faster.
Now to the technique for getting a smooth curve on the fin again. Don’t just sand the immediate area that is damaged but the 5 cm on either side of it in long broad strokes (to get a gradual curve) moving from one side of the fin to the other every two strokes or so (to make sure the leading edge doesn’t become flat but rounded).
As for the trailing edge, here is my advice on that. Obviously a sharp edge will make for a clean separation of the water from the fin and reduce the risk of a spin out. However, do not make it so sharp as to be able to cut flesh. I have had the painful experience of accidentally kicking the fin when hastily positioning the kit for a waterstart and have to get stitches on the sole of my foot. That same fin could just as well cut another part of your body in a wipeout or a stranger in a collision.
In short, have a narrow trailing edge on the fin but not so sharp that it cuts.