Let the epoxy totally harden before sanding. If you're going to do filler coats, go ahead and do that, but dont sand that edge. If you lost your window of opportunity to add filler coats without sanding, wait until the epoxy is totally cured. You can also work around that spot. Once the epoxy is crystal hard, you can sand the area, but use fine 150 grit paper. I have butted pieces and had them totally dissappear when wet out.

If you sand when it's leather hard your screwed. You essentially introduce semi hardened particles into the raised joint and It will never wet transparent.

Your chances of getting to wet totally transparent are probably about 85%. I usually use joints on the inside - usually because I'm using up glass but usually don't want to gamble on the outside.

good luck