Interesting... My homemade Deans arming switch arced too. In fact, my smaller planes that don't use an arming switch at all also arc when I connect the battery. Next time you go to your LHS, wander over into the electric train part of the shop.... assuming you have an LHS that has an electric train department. When there, ask the guy where he keeps his rail cleaning fluid. I've had a bottle of stuff called Railzip forever. It works great for cleaning electric contacts. I wouldn't use emmory board. It is too abrasive, IMHO.