most places for antique plates the vehicle has to be in near showroom condition, not just "old" - even no stains on the upholstery or oily engine, etc - understandable
if you're dead set on getting antique plates instead of regular ones [that don't require a paint job], ya just hafta play by the rules ... you might try getting some real cheapo water based interior latex and later scrub it off, but if you get the antique plates that way and get caught later, could be in a hellofa mess
Bill
