Get a piece of soft rubber, like the kind used as the backing on some sanding disk, put your sand paper on it and work in a figure eight motion. The rubber will allow the sand paper to form up around the edges of the part, which will deburr the outer edge. You always want to work in a figure eight motion so you don't get excessive wear in any one spot. Years ago I used to rebuild hydraulic jacks and we would deburr the body ends and then do the the same thing only on a flat hard surface to lap them back flat. The figure eight motion really works to eliminate any wear spots. Hope this helps.