Can I use your kit to repair the black edges on an old mirror instead of stripping the whole mirror and redoing it?

We generally do not recommend that you try to re-silver a select area of a mirror. The process of preparing the area for re-silvering can damage the curerntly intact silver. Additionally, the chemicals cannot flow as freely as they need to in a small area, especially at an edge. The best results will come from re-silvering the full piece.