Clearcoat that is dull can be polished back to a nice shine in most cases. A polish of the correct grit and type is needed. This one works well for us, others are available.
A "dual Action" polisher is the easiest tool to use. Polishing can be done by hand with much time and effort. A single action rotary buffer tool can be used but needs an experienced hand to avoid damaging the paint.
First clean the whole vehicle to remove all contaminates. High pressure water is used to clear all grit from crevices.
Then as an optional step would be to use a clay bar to remove stuck on contaminates.
Follow the instructions for the chosen polish; usually this involves polishing one small area at a time. Clean off all the dried polish. As long as the paint is intact this is all that will be needed. Applying new clearcoat by itself is not a common repair.