I have painted several mobile antennas in the past to reduce visibility...mostly with flat black Krylon. Just test the paint before you spray and be sure that it doesn't have conductive metal particles in it (spray onto to a non-conductive paper and then measure resistance at all scales of your VOM. If in the 100's of megohms you still be ok. If a direct short....masking will cure.. To be on the safe side, mask off your loading coils before spraying to not cause a change of impedance to the coil or a short around the coil from whip section to whip section. . A brush-on enamel should be ok too and may be a little more durable too as you hit insects and road grime.