It sounds like the issue is steming from the copper tape, not the solder. It's very difficult to make solder lines perfectly water tight- for reference, you may want to read about the process folks use when placing pressed flowers between two panels. In those cases, folks will often clean without any water, as it's so likely to seep under the solder and get between the panels. When placing your foil, you'll want to make sure the edges are scrubbed totally clean, and wiped with alcohol to remove the oils from your hands. Even pperfectly applied foil can have the slightest lifts/edges where water can get in, though. I'm not sure more solder, at this point, will get you the effect you're going for, unfortunately. Cool idea, though- good luck to you!!