Durable method of wall-painting using watercolours on wet plaster (6)

I believe the answer is:

fresco

(Other definitions for fresco that I've seen before include "Painting done in wet plaster" , "Painting using watercolours on plaster" , "Painting style" , "Artistic work" , "Forces (anag) - wall painting" .)

I've seen this clue in The Mirror.

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