First the caveat, I am not an EE or antenna guru.
That said, the RV-12 plans call for the Garmin GPS antenna to be mounted under the cowling which would be painted. I would think a thin coat of something with no carbon, lead, or metal flake would be fine. If it were me I would make a small cardboard cover for the antenna and paint it with a very thick coating of paint and see if it affected signal reception. Depending on how your glare shield is covered, you could velcro a thin fabric cover over the top of it.