We recently tried out Rustoleum’s NeverWet, a new spray-on hydrophobic coating for all kinds of surfaces. It's not recommended for electronics, but we decided to (safely) experiment with some circuits!
DO NOT ATTEMPT THIS AT HOME! Waterproofing electronics is not the intended purpose of NeverWet, and Adafruit is not affiliated with Rustoleum. We're professionals who followed proper safety precautions when conducting the following experiments.