As your vehicle gets older, several things can happen. The paint oxidizes, manifesting itself as a  kind of cloudiness in the colour.

If your vehicle is exposed to sunlight a lot of the time, the surfaces will oxidize faster than if it is kept indoors.

The paint also collects contaminants like road tar, but other contaminants embed themselves in the surface.
Everything from insects to iron particles from the road. Iron particles can give you a heart attack, because they rust on the surface
and make you think that terminal rust is already setting in. Such particles can also make the surface feel rough to the touch.

Paint can also be etched by bird droppings and industrial pollution and should you accidentally drive over a newly painted white
line in the road, getting that very sticky and tenacious paint off your lower bodywork can be so much easier if there's a coat of wax
in place.

Finally, the paint gets scratched and chipped by stones off the road and inhospitable door banging by the car parked next to you.
Big scratches, defined as something you can feel with a finger nail, are best left for a professional to clean up, but the finish will also
have many fine scratches caused mostly by those automatic car washes with the mobile brushes. 

To carry out a clean up job yourself, you will need a few special purpose cleaners and waxes from your local auto parts store,
but the shelves are filled with a confusing array of products. So let's assess what you need to do to the vehicle before you make
your selections.

Washing is obviously the first step.

For this,  you will need liquid soap especially designed for washing cars. Dish washer detergents strip wax finishes.
A car wash soap will lubricate the surface and float the dirt away as it is washed off, instead of rubbing it into the surface, .

Road tar, tree sap and dead bugs will need another cleaner.

Tar deposits often contain abrasives that will scratch the surface if they are rubbed in.
Spray the cleaner on and let the deposits soften so they can be removed gently.

Dry the car with a chamois by squeezing the chammy dry and laying it on the surface.
Gently drag the sheeted chamois over the surface and let it dry.

Run your hand over the surface. It should be very smooth. If it feels rough, then there are deposits still impacted into the surface
that can be removed with a mild compounding paste.  Rub the paste into the surface very gently and remove it with a soft material
such as a well washed cotton undershirt.

The compound also removes wax, so new wax will be need to be applied. The more severe the oxidization, the more aggressive
the abrasive wax required. Rub it in so that the surface is polished. Do all this work by hand. Motorised polishers are for professionals,
(just like sanding you parquet floors) - don't even think about doing it yourself.

At last it is time to give the surface its final coat of protection. Carnauba based waxes are the best.

Stay away form cheap products that use chalk as the polishing media, which will have you going round for hours picking white
deposits out of every nook and cranny. Teflon based products are not much good either.

Never be tempted to use wax on your windshield, it will ruin the wiper blades and smear like mad.

On the side windows and rear windows, that do not have wiper blades, a touch of wax does no harm and is more effective
than Rain-X. (Which is not really saying very much).

Note that factory wheels are often powdercoat painted and should be treated like a painted surface.