One of the most frustrating things restorers have to face is the "surprise" hidden under a fresh coat of paint. You go out looking for a reasonably rust-free project car and the seller tells you that only thin films of body filler have been spread (over properly welded patches, of course.)
You buy the car, take it home and strip away all the paint, only to find old rust areas that were improperly repaired and a thick layer of body filler hiding everything. That means you will have to repair it correctly and it also means you paid too much for the car.
Next time you go out looking, leave that magnet home. Magnets don't really tell you there's a thick layer of filler unless there's no metal behind it. Instead, grab that Stud Sensor out of the toolbox. Stud Sensors are densitometers. They detect changes in density of materials, so if you slide one over a bad Bondo job its little lights will tell you there's something amiss.
You might still want the car, but at least you won't pay too much.