Use and Care Instructions
Remove magnets about once a month to clean the surface to prevent any potential build up.
Do not apply magnets to your vehicle if your vehicle recently underwent a paint or waxing job. Check for the proper cure time of the coating before applying magnets.
Make sure to apply magnets to a clean surface. Any particulates under the magnet may rub and cause damage over time.
Here is a great article from Signs.com on vehicle magnet proper usage and care.