Hockney shows off his 'wishy-washy' watercolours

David Hockney has unveiled one of a new exhibition of portraits that includes his first watercolours, a medium he previously scorned as "wishy-washy".

Hockney chose a portrait of the opera impresario Sir George Christie and his wife, Mary, as the first painting to be shown yesterday. The couple were two of a series of friends who posed for the artist in the same swivel chair, during a year in which he produced enough material for the entire exhibition, Paintings on Paper.

It is the first time Hockney, 65, has worked in watercolours, which he has previously derided as used by amateur painters. He now says he found more depth in the technique after going to Iceland and Norway to take advantage of 24-hour daylight.

The artist got his inspiration for making watercolours after visiting an exhibition of the French watercolour artist Thomas Girtin, who died in 1825 aged 27. He said he had to learn a new way of painting but found watercolour could capture scenes even a photographer could not do justice to.

As well as portraits, the exhibition includes 4ft-by-3ft landscapes made using four pieces of paper, which Hockney painted in sittings of seven hours, using no sketches.

The 42 paintings will go on show at the National Portrait Gallery from today to June, and at the Annely Juda Fine Art gallery in London from tomorrow to 1 March.