I have magnets that are normally on my car. I took them off to wash my car, and there’s a film left on my car in the shapes of the magnets! Any reason why this happened and any suggestions on how to fix/prevent this for the future?
The film is probably a coat of wax that has been worn off on the rest of the car, but protected by the magnets. I’ll bet waxing the whole car would make things look uniform again.
Those magnets don’t make a perfect seal between the magnet and the car paint. Moisture and dirt on paint will leave a mark like that.
To prevent this…Don’t put any magnets on your car.
That’s what my dad thought, too. Thanks 
I’ll try that one first!
Yes, in an earlier life I had magnetic signs on the doors of my 68 Dart. When I changed jobs and took the signs off, it had bubbled the paint a little even. Recommend no signs or a good extra waxing under them.
Eventually the paint will fade differently on the covered and uncovered areas. At that point, you won’t be able to do anything about it.
And any of those magnetic thins will also prevent fading under them leaving the owner with just one choice, A complete paint job.