Waxes are merely barriers. They only give short-term protection if underlying rust and occluded salt is not removed.
A thorough preparation consists of sanding all accessible areas, and treating with a phosphoric acid conversion coating in all areas. There are also tannic acid treatments available. However, IMO they are not as effective as phosphate conversion. After 24 hours, excess chemical is washed off with clean water, and allowed to dry. THEN apply any wax coating.
Unfortunately, rust never sleeps. Once salt has gained access to inaccessible areas such as crevices, what corrosion engineers call a differential oxidation cell is initiated, which is extremely difficult to reverse.
Sometimes, it is better to sell the car before crevice corrosion becomes evident, because it is occasionally a structural issue as well.