Talk to Dan Ballin. He painted his airplane with good results.
BTW, external antennas with internal ground planes will not be affected by metallic paint. A ground plane doesn't need to "see" as it's only used as a reference. Any internal antennas could be affected by metallic paint, but I have heard some GPS antennas working under that type paint.

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Jon Hadlich
AI Systems
(541) 815-7381