A visual inspection of the paint is the first thing you should do. Is the paint thin? Is it stained/water spots/oxidized? Once you determine what condition the paint is in then you look at your options to restore it.
If the paint is thin; then a good washing/cleaning of the surface, clay bar and wax, will make it look better, but eventually it will need to be repainted.
Paint thickness is good but stains and water spots are present; Use a wax and grease remover, then a good washing/cleaning, then you could use the clay bar, then an orbital buffer with a cleaner polisher wax, and then the carnuba wax. This should bring back the paint fairly well.
Paint is good but heavy oxidation and staining small scratches; Use a wax and grease remover, then a good washing/cleaning, then you color sand the paint with 1500/2000 grit sand paper, then an orbital buffer with a cutting compound, then polishing compound, and then the carnuba wax. This should bring back the paint to like new condition.
As stated, paint condition will determine what your next step(s) should be.
