There are different strength magnets (and different sizes). I wish the one I had was a little larger (which would make it a bit easier to handle), but I don't think I'd want one much stronger. I think the one I have exerts about 200 lbs of force on the ball. I think it works "quick as a flash" if (a) it's a relatively shallow dent or the brass is relatively thin or soft, and (b) you've developed a bit of skill with it. Otherwise, rather than "quick as a flash" I've been going for "eventually works with no damage". The only horns I've used it on are the old Martin baritone and the Buescher tuba. My impression of both of those is that the brass is somewhat harder than on most contemporary instruments. At some point I will try it on the Cerveny red brass, but that's not really pressing and I want to be quite sure I've acquired the right touch since that horn is REALLY soft.
Bigger badder magnets cost you more bucks (at a rather alarming rate of increase of bucks per unit of increased strength).
It seems initially a quite intimidating thing to try, and I delayed for some time, even after I got the materials. However, I started on an instrument that I could have damaged without much penalty and moved to one that wouldn't be a huge financial loss if I screwed up. If you read some of the threads on TubeNet you will see a litany from several folks of utterly dire warnings about the dangers and ease with which you can mess up your instrument or harm yourself. But I found that, at least so far in my case, these warnings are about on the same level as others from various folks (most often repair techs) about such things as venting a piston valve (truly trivial if you're careful), lapping a slide, soldering on water keys, resoldering and aligning valve slides, etc., etc. This is not rocket science, and if you have some mechanical ability and can work with a degree of precision, take care, and follow reasonable directions, you actually can do a lot of this yourself. But it's not everyone's cup of tea. You definitely CAN get hurt with these magnets (or with a torch, or a drill, or a saw, or various chemicals). If you happen to have a lathe and a milling machine, you can do much more
. We almost have my son's 1934 lathe working correctly, and once we get the motor back onto his 1974 mill (both acquired used at outrageously low prices), I may turn my attention to even more adventurous projects.
Dan Schultz has been a huge help to me in my (so far quite meager) efforts in brass repair and restoration. And so has Jacob Medlin (young custom horn maker in Greensboro: medlinhorns.com) who provided great information for installing Amado water keys. These guys have been great about sharing information about materials, supply sources, and techniques. My next move for the tuba is to replace the valve guides. I was really puzzled about how to do this and thought it might require brazing extra material on or even more complex operations to retrofit more modern guides (like Yamaha or Jupiter). Dan said (I paraphrase) "Well, those brass guides just screw into holes in the piston with 3-48 threads. Just unscrew them, get new guide blanks from Ferree's, shape them, and install them." Sure enough, I got a pair of pliers and easily unscrewed the first valve guide on the old Martin. Kraus actually lists Delrin valve guides -- but apparently they are "out of stock" for a while. Guess I'll go with the brass.