I'm not sure if this helps but many dealers sold the cars new with dealer 'fitted' tar on the underbody of the car. The cars that I have seen with this have for the best part very good rust free underbodies. Maybe that's because they are in Australia (which is a lot like California in terms of climate) but if you could find out what sort of tar was used, or use a similar product, that might work well (but it will cover the original paint colour).