there might have been a thread on this already...i found a handfull of fins either stock or put away years ago cuz it couldn't point or spin-out and just plain crappy and i just realized all of them have very smooth services just like newly painted and waxed and i compared them to the fins that i like and use and the latter have a slight rough surface..myi thought smooth surfaces are fast but i remembered someone telling me to at least sand a bit to "bite" on the water...is this true cuz if it is i'd start doing and using them a pity they have really nice profiles...if so do you sand with what gritand up or down or leading to trailing and not sharpen the edges

Hey Johnno,
Trying to justify a bit of fin sanding
Wifey away?

I remember Mal Wright said you can only get away with a polished surface if the foil is perfect, otherwise they'll spin out.

I use 400 wet and dry on mine.
rub parallel to water flow.
keep the last third straight with no convex section close to the trailing edge.
The trailing edge can be half a mm thick, but keep it flat not rounded, if it hums, make that flat at a bit of an angle.

If you want to be able to push against them to get upwind, don't thin out the leading edge too much.
But if you want them for speed downwind, don't leave the front edge too thick.

OK, if you put a straight edge on the trailing edge in the direction of water flow. it should be flat for about the last third, or even a little bit hollow close to the trailing edge, if you can "rock" the straight edge on the last quarter of the fin, I'd sand that bit flat.

The idea is to get the water flows off the trailing edge from both sides as parallel as possible. The faster they converge the more turbulence and drag created.

more turbulence/drag = slower.