I know of an RV-4 owner that patched some cracks in his rudder skin by making some small discs of 0.016 aluminum sheet, and gluing them on over the stop-drilled cracks with proseal (sanded paint off down to the bare aluminum first over the crack areas) and then repainted the areas that were patched. It's been several years and several hundred flight hours later, and the cracks have not emerged from behind the disc patches, so apparently it worked.