The general recommendation appears to be outdoors (blue) painters tape to either form the numbers or tape down magnetic numbers (I have joined the second camp). I have been wondering about electrical tape as an option, as to me it seems generally more robust. Any reason not to use it?
I use outdoor masking tape( at least on the front edge of an 81/2 x11 magnetic sheet with numbers printed on and the corners of the sheet radiused). Seems to work well in the cold/wet. Last time out when temps were warm I didn’t use tape at all.
I suspect the electrical tape might leave sticky hard to clean residue but since I haven’t used it I don’t know for sure. The outdoor masking tape comes off cleanly.
I tried both this weekend, on on each side of the car. The blue painter’s tape worked well, with little residue, and no dirt adhering to the leading edges. It did have a bit of a lift at one end. The black electrical tape looked better (black background magnetic label with white letters on a black car), stuck a bit better, but did have a line of dust on the leading edge, easily cleaned off. Since the car needs a bath anyways after the DE, either works.