The Waterat foils - and the Lindsay foils we used to use - are normally shaped to a NACA 00 family section. These sections are much more forgiving of nicks and dings - and our less than perfect steering - than the laminar flow sections. None the less, all the performance improvement you get from a great centerboard is only there if you keep the centerboard in near perfect condition. When was the last time you checked you centerboard for nicks and dings? Any imperfections can increase turbulence and drag. Apart from robbing you of speed and pointing, a damaged centerboard can absorb water, weakening it and eventually leading to it breaking. Now that is a performance (and dollar) robber!
However, as Larry points out, wet sanding the foil removes that very tough shiny finish, revealing a softer layer underneath. Once you wet sand a board, you will have to keep wet sanding it, as it will scratch more easily. A wet sanded surface also gets dirty more easily, and therefore requires more maintenance than the smooth shiny finish. Eventually, the foil will have to be refinished, and you can start all over.
Waterat does NOT recommend sanding your hull!
If you are interested in more discussion of surface finish on foils, check out C. A. Marchajís books - the first is Sailing Theory and Practice, Dodd Mead & Company, 1964, the second Areo-Hydrodynamics of Sailing, Dodd Mead & Company, 1979 - and Frank Bethwaiteís recent book, High Performance Sailing, McGraw-Hill, 1993. Long time Annapolis area 505 sailor Bransford Eck also wrote two articles discussing 505 centerboard cross sections, planforms, area, and finish, in the '70s. Many long time 505 sailors have copies. Much of the discussion would be applicable to other centerboard classes as well, though the resulting centerboard would be different.
Dents and dings in the leading edge are a little more challenging; you want to end up with exactly the same shape on the leading edge - this is critical! Cut or file away any broken fiberglass or wooden fibers, and then use epoxy and filler to both seal the wood, and fill the dent. I find that West epoxy with microballoons mixed in works well. It is strong, yet files or sands easily. Being softer than the polyurethane finish, you should be able to sand or file it to fit, without taking too much paint off around the damaged area. I usually shape the repaired area using a medium file. I find that if I do not press too hard, the file does not scratch the polyurethane on the surrounding area, but does easily remove the excess filler material sticking above the surface.
Trailing edge damage is easier to fix. Clean out up broken glass or wood fiber, then find a piece of semi rigid plastic - a small piece of mylar is perfect. Tape the plastic to one side of the blade, so that you are creating a mold for one side of the damaged area. Then using an epoxy and filler mixture, fill in the damaged area. Trailing edges are inherently weak, if the area you are repairing is much more than a thumbnail in size, you should reinforce the epoxy with a little fiberglass cloth. Simply lay the cloth in, and make sure it is saturated with epoxy. The low density fillers like microballoons reduce the weight you are adding in the repair, but this is usually insignificant for a small repair. Low density fillers do make the repair easier to sand and file to shape. High density fillers like microfibers make the repair stronger.
I use the same mylar approach to repairing a CB bolt hole. After Iíve cleaned away all the broken material, I tape one side of the bolt hole closed with the mylar, and then put epoxy and filler in the other side. Sometimes putting just the right amount of filler and a second piece of mylar on top results in a repair that needs minimal filling and sanding; you just have to drill the new bolt hole through the filled area. The CB bolt hole in the centerboard should be lose enough that the board can gybe from side to side without being constrained by the CB bolt; it should not be any larger than it needs to be.
From time to time, I build back up the gybing head of the centerboard. I clean up the worn area with sandpaper or a file, mix a batch of epoxy, using a high density filler (Iíve used both microfibers and carbon powder for this), and then applied it over the worn area. I shape it immediately with a clean trowel, scraping off much of the filler, but leaving the filled area almost flush with both planes of the gybing head (you should see a distinct ridge where the two planes meet). After the filler has cured, I carefully file and then wet sand the repaired area.
I have a 12 year old centerboard that is as light, stiff and fast as a new one - it has probably been refinished two or three times.
Centerboard and rudder repairs can take time, but perfect foils give you better speed and pointing; critical to racing success.