Even non-metallic paints can have an attenuating effect on microwaves, especially at K band and above. Unless you know the composition of the paint or it has undergone testing, it's hard to say for sure what effects it will have.
Side story: I used to carry a gadget that monitored my exposure to RF at sites where it was a hazard. The rep from the gadget's manufacturer said the coloring they originally selected for its plastic housing attenuated everything from 75 GHz to the gadget's upper limit of 100 GHz (yes, you read the units correctly. They had to go through several kinds of coloring to find one that didn't cause problems.
--Mark