Deep scratches, scars, and other breaks in the finish tend to attract dirt like magnets. Polishing, either with water or oily cleaners, will simply rub the dirt right into the scratches:
Modern instruments may be finished with a catalyzed polymer coating that is quite inert and unaffected by perspiration and skin oils. Nitrocellulose lacquer is much more reactive, however, and needs a bit of understanding to avoid difficulties in cleaning or polishing.
The finish on the back of the neck gets the most handling of all. As it is handled, the lacquer may react to your skin chemistry and become soft, increasing the rate at which it wears:
After enough "mileage" the simple act of playing a guitar will wear the finish off in the areas that are handled the most:
It goes without saying that you can't clean the finish if it's worn off!
The area of the top where you rest your forearm is particularly vulnerable to becoming softened as it reacts to your body chemistry:
Like the neck finish, it wears prematurely when it becomes soft, and it may eventually look like this:
Now, these are extreme examples of long term wear, for sure. But, the softening of the finish may be noticeable even when a lacquer finished guitar is quite young. If you notice a dull area where your arm crosses the face, don't be to eager to try to polish it vigorously. You may have a slightly softened finish, and it would be a good idea to take it to a professional luthier for some advice.
Areas with broken finish are best left alone. Rather than using polishes or solvents, simply wiping with a dry cloth is about the best you can do to maintain these areas:
