In general no. I assume you're talking about run of the mill NEMA 1 or 3R. In fact for outdoor gear, I would highly recommend it. The factory paint will not last long.
Thanks Larry. I contacted Square D and they are going to email me something that says it is ok. I have saw it done everywhere, but the other day someone brought it up as to was it still listed. You see it on emergency panels and such all the time. In fact they make red EMT now.