Vehicle Graphics  1
Front door magnets are the most common.

Vehicle Graphics : Magnetic Signs

The bodywork on most cars, trucks, small vans, etc. is made from ferromagnetic material, which means that magnets and magnetic materials will stick. A flexible magnetic sheet material is available, which can easily be cut to any size. Vinyl graphics can be adhered to the magnetic sheet, which can then be magnetically attached to the vehicle as an effective temporary or permanent advertising sign. These types of signs are most suitable for smaller vehicles. In many instances, larger vehicles have bodies made of fiberglass or aluminum and magnetic material will not be attracted to them