Sticking anything to a textured surface is never an easy task. You could try warming up the surface with a hair dryer (and the sticker) prior to attempting the job. Work quickly as the metal will cool fast.

Don't know how warm it is in your part of the world at the moment, but I've always found vinyl graphics stick best at temperatures above 25-30C. The heat gives flexibility and allows the adhesive to mould into the texture of the surface, filling the pits, rather than just sticking to the high points.

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