Many cars today have alloy wheels instead of removable wheel covers (hubcaps). Alloy wheels are attractive and can look great for many years with the proper care.
That ugly black residue that collects on the wheels - the front wheels in particular - is brake dust coming from the brake calipers. There's nothing you can do about getting this gunk, but you should make a point to clean your alloy wheels frequently to prevent the black dust from staining the nice silver finish. Most alloy wheels have a clear-coat finish applied over the metal, so occasionally use a good car wax/polish to keep the wheels in top condition and repel the dirt and brake dust.
Even with the best of driving care, you will occasionally rub up against a curb and possibly scratch or chip the edge of your alloy wheel. It's a good idea to repair the chipped area before the exposed alloy becomes permanently stained or oxidized. Auto parts stores sell a clear-coat touch-up kit just for this purpose.
Clean the wheel thoroughly, then dry it completely, especially in the area to be repaired.
Smooth the repair area using a fine (400-grade) emery paper or scouring pad. Carefully feather the edges of the clear-coat surrounding the affected area, then wipe clean with a dry cloth.
Use the brush which comes with the touch-up kit, or a fine artist's brush, to apply the clear-coat. It's better to use many light coats which will eventually blend evenly with the surrounding area.
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