My parents drove a Fiat 2300 Estate in the early 1970's, which was Magnolia coloured with some rust on the sills.
My Father decided he wanted to paint it Yellow, so we were rubbing down the paint prior to respray, when my sander went through the paint to something white and powdery
The tops of both front wings had been "repaired" with Polyfiller and layers of cotton gauze.
We tried the magnet test and there was nothing magnetic for at least a couple of hand spans, both sides.
The lower wings were slightly magnetic, but that turned out to be chicken wire embedded in the Polyfiller
Dad had always used the same garage, that sold him the car, for the MOT's....