Removing 83 nails, he saw a flash of luscious, almost luminous, green painterly brushstrokes appearing in one corner - the first glimpse of a superbly preserved portrait of the young A.J. Munnings.
Research has revealed that Alfred James Munnings Reading c.1910, shown here, closely relates to The Sonnet, a lost work by Harold Knight (Laura Knight's husband) exhibited at the Royal Academy in 1911.
Why it was hidden is a mystery, but theories focus on the relationships formed when Munnings and the Knights lived in Lamorna, Cornwall before the First World War.
Did Harold Knight's dislike of Munnings lead him to dispose of the painting only for Laura to 'rescue' it and hide it beneath one of her own works? Or was it covered to dampen painful memories of the suicide of Munnings' first wife, Florence, in 1914?
Estimated at £30,000-50,000, it will be in Christie's December 16 sale of Victorian & British Impressionist Art.