Are you mixing up two different types of stealth technology? Radar stealth requires absorption of radio waves by a special coating but sonar stealth for ships uses the rubbery absorption of sound "pings". A rubbery paint coating on a metal automobile probably would do little to reduce State Trooper radar reflection, although the question is interesting in the case of a fiberglass car. As suggested above it is probably best to just use a flat black, add ghost flames and enjoy the visual effect. Probably the lump of iron in the motor will provide a radar reflection even with a fiberglass car and a metal car with some smooth surfaces will reflect radio waves nicely. After all a trunk mounted CB antenna uses the metal trunk lid to form the bottom half of a "P-wave" since most CB antennas use a half-wave whip type sending antenna .

Don Shillady
Retired Scientist/teen rodder