The scene shows an idyllic, dewy morning, with boatmen at work on the lake – and, pale in the background, the palatial country house where the artist, JMW Turner, kept his brushes.

Petworth in Sussex was the seat of one of Turner's greatest patrons, the third Earl of Egremont. Now curator Andy Loukes has gathered scores of Turner's Sussex paintings for the first time at the estate, on loan from museums such as the Tate, the V&A and the British Museum, and other collections – including a view of Bodiam Castle that has not been exhibited since 1862.

Some were actually painted in the mansion, now owned by the National Trust, where Turner had the run of the house, annexed the enormous library as his studio and kept the spare key in his pocket. The artist was famously secretive, and Egremont was one of the few admitted when Turner recognised his distinctive step on the stairs.

The sculptor Francis Chantrey, another regular house guest, is said to have become so curious about what was going on that he imitated Egremont's gait, and managed to rush into the studio before the door was slammed again.

Although most of the house is shut up for winter, visitors to the exhibition will also see the rooms that still hold the 20 Turners that Egremont bought, and the library, not normally open to the public.

His earliest work at Petworth, a pencil sketch of the house made from the park in 1809, is on display beside the grand oil painting that became his first major commission from Egremont. Although the churchyard picture is almost 20 years earlier – Turner's first known view of the county – Loukes believes the London boy probably had not yet set foot in the rolling Sussex countryside, but copied it from another picture. When he did discover the joys of Sussex he clearly loved it: after Yorkshire and Kent, it was the county he most often painted.

The exhibition also has a group of his intimate little watercolour sketches on blue paper, also painted in the house, including a hazy view of a naked woman in a bedroom – one of the erotic paintings that so shocked his puritanical patron and critic John Ruskin that he claimed to have destroyed them all after Turner's death.

Some of the sketches are still owned by the Egremont family, including the one known as "the spilt milk picture": Turner, clumsy with anything except a brush, knocked a milk jug over the earl's niece and made a little painting of her as an apology.

Turner's last Petworth painting, in 1837, showed Egremont lying in his coffin in one of the grandest rooms. His heirs lost no time: within days of his death, Turner and the rest of the rabble of artists were turned out of the house forever.

Turner's Sussex runs from 12 January to 13 March. Booking on www.nationaltrust.org.uk/petworth-house