Heavy deposits on exhaust valves - MC engine

[Eyvind Spangen]

I'm currently doing a valve job on a MC head due to the previous owner 
bending intake valve #4 and #5 when changing the timing belt. Now, all the 
exhaust valves have rather heavy deposits, both on top and on the part into 
the combustion chamber. How can this be removed? They are rock hard too..

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E. Spangen - Norway, Europe
'88 100TQ
'86 80TD