V-gouges are slightly difficult to sharpen. The inner part of the V is, in particular, difficult for me to get. I have read that you can actually use a very high grit sand paper, although I'm a little skeptical of doing this. Has anyone ever tried this, and if so, does it work?
I have sharpened similar tools. I very rarely will ever touch the interior cutting surface of a gouge, except to remove and burr that may occur. I typically do this with a small flat file. I believe they are called jewelers files and came in my pack of rat tailed and shaped files specifically for this purpose. Even when affixing sandpaper to a metal ruler or sheet of glass, it's hard to ensure even shaping/sharpening due to the slightly uneven texture of sandpaper. I once used sandpaper (as a young teen) in a vain attempt to sharpen some tools, which were quickly in need of a professional sharpening. I cannot suggest it.
Make the small investment in some nice, fine, small files for burr removal, and sharpen the bevel as you would any of your other tools.
The traditional answer is to use slipstones to sharpen the interior the 'v'. As already mentioned files will work and if you are curious about using sand paper (also as known as the scary sharp method), you can create a little strip of wood that fits the interior shape of the 'v' and glue sand paper to it. Now you can use this sand paper block to sharpen the interior.