I remember reading on seabreeze a while back (but I can not remenber now) that sanding the 1st 3rd of your Wing with 400 grit sandpaper can make a layer of air help reduce the drag on the wing? Does anyone know if this correct & on which surface - top or bottom or both?:-)

For the speeds we travel at not really worth bothering, but the kite foil racing guys are right into it.. sanding in the direction of water flow from leading edge to trailing starting with 400-600 and finishing with 800-1000
the idea is to creat a surface boundary layer of water particles on the foil that the other water particles can flow over with less resistance.

Yes, a rough surface (400 to 600 grit) traps a water layer, so that you have only water on water friction, which is less than water on foil surface. Note that a surface sanded to 600 will still feel "smooth" to the hand.

It is easy to check: throw water at your foil, the water must form sheets not beads.
See:
boardlady.com/fast.htm
www.standupzone.com/forum/index.php/topic,7044.msg62971.html#msg62971
www.sciencedaily.com/releases/2014/01/140117153637.htm

Select to expand quote

colas said..
Yes, a rough surface (400 to 600 grit) traps a water layer, so that you have only water on water friction, which is less than water on foil surface. Note that a surface sanded to 600 will still feel "smooth" to the hand.

It is easy to check: throw water at your foil, the water must form sheets not beads.
See:
boardlady.com/fast.htm
www.standupzone.com/forum/index.php/topic,7044.msg62971.html#msg62971
www.sciencedaily.com/releases/2014/01/140117153637.htm

Ok thanks colas but do you think if sanding across the wing or leading edge to trailing edge is better?:-)

I have noticed small speed increases by sanding to 6-800 grit. The biggest speed increase was in downwind SUP racing, where small changes in drag translate to large changes in pace. At the speeds of most surf and wing foilers it probably doesn't matter.
Where sanding does make a big difference is in ventilation. In my experience, sanding the mast and wings can make a major reduction in crashes caused by ventilation and help with reattachment after ventilation. 800 grit seems to be the best, but there are diminishing returns above 240. I don't bother with sanding direction and it has worked great over 30kn. I suspect the reason for improvement has to do with making the surface even and smooth more than the texture it creates. Most foil parts come with a clear coat that has an "orange peel" finish and imperfections around the leading and trailing edges. Any amount of sanding should help even out the surface.

Thanks for sharing a pragmatic view to foil finishing FoilAddict, your approach sounds like something I'm up for doing.

I think this guy has some really good info to share, but I'm not racing so much of it is well past my point of diminishing returns.