I am finished with old painting technique (7)
I believe the answer is:
impasto
'painting technique' is the definition.
(I've seen this before)
'i am finished with old' is the wordplay.
'i am' becomes 'im'.
'finished' becomes 'past'.
'with' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'im'+'past'+'o'='IMPASTO'
(Other definitions for impasto that I've seen before include "Technique of applying paint thickly" , "Good covering of paint" , "Style of painting" , "Method of painting" , "Thickly-applied painting technique" .)