Usually any old kind of household degreaser will remove brake dust in the
worst case. Normally you should just be able to use detergent and water.

The wheel will have to come off, then you can press the center cap on and
install the wheel again. The close tolerances between the center cap and
the big axle nut on the rear don't allow the plastic teeth enough space to
expand and pull out like the front.

Or, you could take the wheel back and ask the shop to finish the job you
paid them to do.

Jim

On Sat, Mar 30, 2013 at 3:10 PM, David M <covrambles@yahoo.com> wrote:

> I have the 14in VW alloy wheels on my 1987. A few weeks ago I had a rear
> tire changed and when the shop did it they omitted to reinstall the plastic
> center cap. Since I could not figure out how to re-install it, I just left
> it off and did not think twice about it. When filling up for gas, I now
> noticed that that alloy wheel (and none of the other three) is covered in
> blackish gray stain. I'm assuming this is brake dust? Does the center cap
> really prevent this or is it just a concidence? And just how to I get this
> wheel looking shiny aluminum again. I tried using toothpaste and it had
> little effect. How do I reinstall the center cap? Thanks.
>
> -David (1987 Wolfsburg GL, Tempe AZ)
>