You don't have to wet sand the whole car, although, depending on lots of variables, it might show. I know some people only do the visible areas ie belt line upwards. I don't see why you couldn't fix a problem such as a run and polish the whole side of the car. I spent alot of time getting reasonably skilled at wet sanding and polishing and do recommend just going to a pro unless you're willing to spend time and money (tools and supplies) getting good at. There is also the risk of damaging the paint job beyond repair.