If I may be so bold as to quote from another forum without the originator being named, here is how he solved his problem.

"Here is how I fixed it. I got an inexpensive ($43, and that's cheap for this kind of instrument) from a site called Less EMF. Then I got on eBay and grabbed a 13" degaussing coil. These degassing coils were originally intended to degauss television screens, which would sometime develop oddball mag fields. I'm told it gave you purple spots...whatever, these are pretty available. I paid $13 and shipping, you may pay more or less...all depends.

Anyway, I could pinpoint the areas of high magnetic activity and then hold the degaussing coil over it for a few seconds and slooooowly move it away. Rechecking showed a reduction, sometimes complete. I had to do it a couple of times in some spots to reduce it to zero. All in all, very simple and painless.

Wow! Was that simple or what?

Why does the airframe get magnetized? Two most common ways are if you electric weld on it you are sending lots of amperes through the structure, and this will make localized fields. Second, if you have a battery mounted elsewhere than on the firewall, don't take the easy way out and ground it through the fuselage tubing. This will make magnetic fields for you. Instead, use a dedicated grounding cable from battery to starter, and bond all the system grounds to the ground terminal at the starter."