Fumed Silica is used in a great variety of paints as a viscocity enhancer, I do not know if it makes the paint more scratch resistant.
B.E.

What about carbon nanotubes? That would seem like a pretty good idea - it has been shown that CNTs can double the hardness of copper.
Only problem would be that if the product ever gets burned or destroyed, you might release CNTs into the environment.

geneiusxie,
I think you will get a more sensible answer if you move your question to forum332: Paint/coatings engineering I have no experience with carbon nanotubes or buckyballs.
B.E.

CNT's will increase the viscosity extremely with the addition of only a few percent.

Depending a bit on what kind of durabilty you want can can add a small particel filler with a high hardness on the mohs scale, this will give you a smooth coating or just add some silica carbide which has a larger particel size which will give you a rough coating.