It almost definitely won't - but there are effects that could be some coatings that would cause problems. It isn't just metal content, there's also the possibility that some coatings could absorb RF at certain frequencies. There are many amateurs who have tried this sort of thing and tested it, so the best bet would be to search for the specific product you intend to use to see if anyone has posted results online. If not, test the antenna before and after to see if the resonant frequency or gain has changed.
When using powder coating, make sure that whoever is doing it knows that the antenna may flex, otherwise the coating could become damaged too easily. Anodizing the antenna would prevent this problem, but may be more likely to alter the electrical properties - especially due to the "skin effect" causing more current to flow in and near the anodized layer than in the center of the antenna.
Other tricks include heat shrink tubing, which will form to the antenna. Some types will become stiff once activated, but will still allow enough "flex", others will retain their flexibility. All are non-conductive, so no worries about skin effect or shielding, they should have little effect on the antenna's electrical properties.