Macbook Air Aluminum Wearout
Not that it will help, but that looks like a 2-coat process, as it's wearing away, the 'outline' is the border between the top coat &amp; the substrate; the patch in the middle is where that base coat has also worn away.  

In short, I'd say not fixable, other than by completely stripping &amp; re-coating... or not viable, rather than not fixable.

moved from comments

Anodization changes the microscopic texture of the surface and changes the crystal structure of the metal near the surface. Thick coatings are normally porous, so a sealing process is often needed to achieve corrosion resistance. Anodized aluminium surfaces, for example, are harder than aluminium but have low to moderate wear resistance that can be improved with increasing thickness or by applying suitable sealing substances.