Valve shim question

[Raldel@domain.elided]

    Can valve shims be milled down and used, or do they have to be purchased 
at the correct size? I think they could easily be lapped down on a piece of 
plate glass, some 400 grit wet or dry paper, and some WD-40. I am wondering 
if this process would take off some sort of coating or if the top of them are 
hardened or something. What is the thoughts of the digest?
      thanks,
   randy long
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