Use magnetic tape for fixing signs i.e on metal surface. 

Magnet Tape is a flexible, polymer bonded product coated on one side with the pressure sensitive adhesive. The magnetic material is made using a special process that orients barium ferrite crystals in the polymer binder during manufacture.

Magnet Tape can be bent, twisted and flexed without loss of magnetic energy. The product conforms readily to irregular surfaces without cracking.

It will not lose its magnetic strength when dropped or struck.

3M Magnet Tape is, indeed permanently magnetic. 3M Magnet Tape has a high-tack pressure-sensitive adhesive layer on one side, protected by a peel-away backing. The adhesive will adhere to a wide variety of surfaces including most plastics, wood and metal. The non-adhesive side may be used to hold small ferrous objects such as screw drivers and other tools. The product also saves time by holding small parts during assembly. 3M Magnet Tape is a versatile holding material, which can be used effectively on both metal and non-metal surfaces.