What I have found is that as long as there is paint on the car enough so that can lightly compound without revealing undercoat, you can almost always renew to great extent. First paint 'scrub' (removes small scratches and oxidation), then polish then seal with wax or urethane covering then you can really bring out the shine.
I use the maguire 'deep crystal' multi (3) step process. First 'body scrub', then 'polish', then 'wax' ie the last step you can use their carnuba wax. If the body scrub dosn't satisfy, take 'white' compound by dupont follow their directions and then follow with Meguiars product.
If you have a random orbit waxer/polisher (change pads often) use it as it will save time and do a better job. Once your paint is satifactory, you can use a product called 'Finish 2001" which is a urethane enriched wax like covering. I have been using for years and it really lasts ( 1 year ) and protects paint. I apply by hand then remove with RO polisher. Change terry cloth pads often.
Hope this helps. Autoparts stores have these products. Finish 2001 is a distributed by Turtle wax. BTW, plain liquid turtlewax once your paint is clear really works amazingly well also.
Ferd