If you’ve ever played a brass instrument, you’ve no doubt worried about accidentally denting it. After all, how exactly does one bang out a dent that’s in one of those tiny little tubes? The answer, it would seem, lies with magnets. Specifically, a couple of very powerful magnets and a steel ball.
As you can see in the guide here, the idea is that you can stick the steel ball into your instrument, and rub the magnets over the affected area (with some sort of padding to protect the metal). This will cause the steel ball to push on the dent from the inside. Once you’ve rubbed the magnets over the area a few times, the dent(s) will completely disappear. What’s amazing is that virtually no trace of the dents seems to remain behind. I can’t imagine how much you would pay a professional to produce these kinds of results.