We are operating at reasonable Reynolds numbers, close to 1 million when sailing at 10 kts. Doing some first order calculations, assuming the flow is laminar (i.e smooth), looking 0.25in (6.4mm) aft of the LE, the boundary layer is 0.098in (2.5mm). Effectively this means any scratches or bumps within that distance would have no effect on the performance of the foil as the water around the foil can't even see them. However, there are A LOT of assumptions made in that first order analysis and you always want to build the best foil you possible can. My #1 tip if selecting a foil for a home build is to go with something whereby the performance doesn't degrade drastically due to imperfections. For example, a laminar flow section like the NACA 63-209 discussed here (http://www.foils.org/hysecdes.pdf) is much more sensitive to surface imperfections and things like kelp than a more traditional foil. The NACA0010 that was designed to operate in a more turbulent flow environment where the boundary layer is even thicker and hence you can get away with even more, besides it is easier to build accurately.