As you've observed, the 'fix' is accomplished with sandpaper.

To prevent this from happening again, don't mix graphite with the first batch of epoxy. If the first layer is so thin that it runs, thicken it with fumed silica (cabosil), micro-balloons or finely chopped glass fibers. That will seal all gaps and joints. Then, if you ar using graphite, put it in the next coat.

PGJ