RE: Fin Maintenance
Namreh has it pretty much right.
If you strike anything in the water, and it damages the profile or the foil surfaces of your fin, then it's a good idea to file/sand out the little nicks.
If the damage is larger (like you hit the rock that Namreh was talking about, and took a big chunk out of the leading edge of your fin, then it's a good idea to get some thickened epoxy (Marine Tex; JB Weld Quik; West Systems and perhaps a tiny bit of glass cloth and fill in the missing chunk, then file and sand back to the original profile.
If there is paint, or logo stickers on your fin, then sometimes this can cause problems, and it's a good idea to sand them smooth with the surrounding profile.
Racers and speed sailors have some different opinions on what the fastest and best surface finish is.
Some seem to think an almost "polished" finish is the best, while others want a very smooth but "matte" finish that "wets out" really well.
Overall, I think the very smooth "matte" finish is the best and this is what we see on the very high end racing and speed fins, from the manufacturer.
Any kind of roughness or change in the profile or texture is bound to cause some "turbulent flow" in that area, and this can lead to "unattached flow" or spinout.
To correct for the "whistling" that Namreh refers to, you need to slightly "blunt" the trailing edge of the fin perpendicular to the centerline of the foil.
The "whistle" is normally cuased by the trailing edge being so thinned out that it can vibrate.
Also, sometimes in the manufacturing or finishing process, small "hollows" are created just ahead of the trailing edge. (it's hard to grind very thin sections without the thinnest part (the trailing edge) pushing away from the tool, so itthe tool removes a tiny bit more just ahead of the edge where it cannot push away).
You can test for this with a very accurate straight edge or a hardened and ground dowel pin.
Check the back of the foil normal to the flow around the foil.
Hope this helps,