Info from "THE BOARD LADY" who has fixed some 6,000 boards.http://www.boardlady.com/fast.htm
"Water is pretty sticky: dip a paddle into the ocean and it comes up with much water attached. Move a board through the ocean – be it surf, windsurf, stand-up, or kitesurf -, and it will drag a substantial amount of water along, which, as it eventually cannot hold on to the bottom any more, emerges behind in the form of a wake. The energy that you generated with your sail, kite or paddle, went into accelerating not just you and your board, but this rather substantial mass of water as well.
To go faster, then, we need to decrease the amount of water “sticking” to your board – “reducing the drag” in techno-lingo, since the best wake is no wake!
In extensive tank testing for world-class racing boats, it was established 40-some years ago that a glossy surface has substantially more drag than a matte one. In tests I was involved in for an America’s Cup boat, we found that simply sanding a glossy bottom with 600 grit paper, reduced the surface friction by about 5% at ½ hull speed, i.e. at about 4.5 knots. Instead of sticking to the glossy bottom, the water molecules would be “tripped up” by the minute ridges left by the sandpaper. This slight turbulence would reduce the thickness of the film of water being moved along with the vessel (the “boundary layer”), and thereby reduce the overall drag.
Surfboards are traditionally finished super-glossy-shiny. Windsurfers, on the other hand, have long since followed the lead of sailboat racers and taken the gloss off the bottoms of their craft.
If you wonder if your bottom is too shiny, throw a cup of water at it: if the water beads, like it will on a freshly waxed car, then the surface tension is high, and therefore its resistance going through the water is high.
To change it to a low-drag surface, sand it in a circular motion with 600 grit wet & dry paper, until water thrown at it runs off in sheets – WITHOUT ANY BEADS forming.
Instead of wet & dry sandpaper, you can also use a “Purple Pad”, a Scotch brand synthetic wool pad designed to burnish metal and take off rust. Again, a circular motion is optimal.
To keep this fast surface fast, wash it now and then with soapy water; and lightly apply the Purple-Pad when the bottom appears yellow.
How big a difference will this make? After a bottom repair a while back, I decided to Purple-Pad the entire bottom of a windsurfing board. The owner noticed with a raised eyebrow, but did not comment on it when he picked up the board. However, he was back 4 hrs later, demanding to know what exactly I had done to his board: a friend, who had always easily passed him before, was no longer able to even keep up with him.
Or this recently from Shawn C: “I purchased an old Hifly 265 poly board that I used a couple of times and really enjoyed. I noticed that the board had a few small gouges on the bottom so I thought that I would sand them down using wet/dry 600 grit as you suggested. I then mistakenly waxed the bottom and had horrible performance on the water.”
I know of someone who painted a KBHF T bar with a super gloss shinny product and it then flew much worse so he sanded it with 600 g and back to normal.