A polished car will stay clean for longer, and will in fact be much easier to clean, even without protection added. This is due to a surface that’s so smooth that dirt has less chance of getting caught in the microscopic hills and valleys.
A smooth surface
Although a surface might feel perfectly smooth, it will in fact be very uneven. If viewed under a microscope, it will shows hills and valleys. These hills and valleys are from very different sizes. They can be bigger then a grain of sand, or even the size of a molecule. The uneven surface gives dirt the chance to get stuck in these hills and valleys. A grain of sand, a dust particle, a water molecule, they can all get caught in the microscopic unevenness of the surface. When these particles get stuck, they themselves can become a barrière that other particles can get caught in.
A polished surface
Polishing a surface will help to minimize the level of unevenness of the surface. It will still not look perfectly flat, but the hills and valleys will be much smaller, leaving much less chance of particles to get caught in the microscopic unevenness. The particles can still lie on top of the surface, but they will be much easier to remove. In some cases a simple rinse can be enough to flush off most of these particles. A weekly wash will be ideal to keep the paint free of contaminants for a while. Note that it is not possible to keep the surface 100% contaminant free.
Causes of an uneven surface
There are many reasons why a surface becomes uneven. Most of them are due to being exposed to the world outside. Oxygen, water and sunshine (radiation) can cause different levels of oxidation. Wind that contains sand particles bombard the surface while driving around, causing microscopic damage. A branch or leave that rubs against the paintwork can cause a surface imperfection that might not be visible with the naked eye. Even drying with the wrong drying towel will actually cause surface imperfections that (after a while) cause imperfections that need correcting. These might not all be visible to the naked eye, but they add up after a while.