I posted a shot like this on Instagram earlier this week about DIY fixes to your surfboard. A lot of people make the mistake of just dropping some resin into a fracture without sanding the area first. But the problem is that it is not going to stick and won’t be watertight. You can think of it simply like rock-climbing. Like you, the resin needs something to hold on to!!!!! So when I am hand-making your surfboard, I sand the fill coat of resin through #80, #120 and finally #150 grit paper. I don’t go any higher than this to ensure the final finish coat of resin “sticks” to your surfboard. If I do go higher, then it risks future catastrophe with your surfboard.
Most “matte” finish surfboards are sanded up to about #400, but that all depends on the preference of the shaper. Gloss surfboards go all the way up to say #2000. After that, I then use a special compound for reducing micro-scratches even more. That’s all gloss is. Making scratches smaller. So, when you put that resin directly on to #400 or higher, it’s not going to stick and not going to last long at all.
So do yourself a favour, sand the area first with say #120 before you apply the resin. Then your field fixes are going to last longer. But don’t go cranking it with something rough like #40 or #60 as you’ll probably sand to the cloth or worse when preparing the area. Think white sandpaper for white or light coloured surfboards if you have access to it. Then don’t touch it with your oily fingers. Wipe the dust off with a microfibre cloth or a piece of paper towel before dropping your resin. Happy fixing!