There are almost certainly going to be some pits or ridges in your joints that would be too difficult to sand out. To fill these imperfections, I made my own epoxy filler out of carbon dust. When mixed in with epoxy, this forms a black, tar-like glue that can be used like putty to fill in the gaps. I made the carbon dust by filing down a piece of a burned wood. I then used a super fine strainer to filter out all but the very smallest particles. Once you have made the thickened epoxy, spread it generously on any imperfections in the surface. Once it has dried, sand it down to the level of the carbon fiber.