Wash car using a mild soap (intended for use on cars) to remove the loose dirt/ grime, follow with a quality clay bar system. (mother's, meguire's, etc.) will probably require more than one bar of clay if the paint has been neglected for decades. After removing all the oxidation/contaminates with the clay bar, finish up using a cleaner type wax followed by a quality wax of your choice. Very little chance of cutting through the paint and keeping the factory orange peel at the same time. Just my