I actually found an answer rather quickly ... [search brought this site up first]. Here is a link to some good information Magnetic Mount Antennas Don't Suck
That article is primarily aimed at VHF and above-------they mentioned 50mhz which is the amateur 6M band. That is nearly twice the frequency of CB. "Here's the thing..............." as frequency goes down capacitive reactance goes up........which is the what -you- might-call "conductivity" between the mount and the car. Add to that "if" you add plastic/ rubber/ cork/ other between the mount, which drives the Xc up even more, the "short answer" is the SWR and preformance will suffer.
Way to revive and old thread LOL
Also keep in mind the SAFETY of mag mounts. "If they let go" due to reduced magnetic holding power, they can damage your or other's vehicles, as well as becoming a projectile that can hurt living things
