I use a dremel tool on the underside, making sure to get all the nooks and crannies.
CAREFULL - Don't burn up the plastic.
Then I use my "Very fine" wire wheel to slowly remove all the years of crud from the cover.
Unless they are rusted or pitted, they usually turn out pretty good.
If you want them to be shinny...
(I don't, I like them dull Zinc)...use a little aluminum wheel polish.
Then, I use soap and water with a Q-tip for the plastic.
I test it first before the cleaning, cause it if they are bad, they are trash - You can try cleaning the points, sometimes that helps get them started.
For a NOS one just use "00" steel wool.
