Countertop Lippage II

Following on from Brian Briggs question regarding acceptable lippage - what is the gauge for a seam that is too unneven to grind flat?

Would you grind a seam with upto 2mm lippage? - this to me seems to entail alot of grinding!

Also with what grit diamond do you do the grinding with?

As I have said before the art of top polishing is unheard of in the UK, but the art of putting in rubbish seams is not!! I am trying here to gauge what is/is not achievable insitu as a fix.

The next time I see a rubbish seam I will see if I can get a picture to post so you can see what I mean.
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David Lauder
www.Marble (http://www.lightemprador.com) lifeuk.com