As we are saying goodbye, Dinos Chapman says: “I’d better go back now and destroy a few Goyas.” He doesn’t mean literally, of course, but after making their names with a dramatic recasting of the great Spanish painter’s etchings Disasters of War, Dinos and Jake Chapman are planning once again to defile Goya in their own unique, playful style.

Where Disasters of War evoked appalling acts of violence, Goya’s Los Caprichos, a series of 80 etchings, focused on the high and low lifes of 18th-century Madrid society. In the only new work to be exhibited at a retrospective of their work at Tate Liverpool opening next month, the Chapman brothers — who have played with some of the Los Caprichos series before — are painting clown and rabbit faces on to Goya’s figures.

If in the etchings Goya was mocking Madrid society, the Chapmans are mocking our veneration that turns Old Masters into relics.

The art world is about to experience a Chapman winter. I meet Dinos at a foundry in Limehouse, East London, where a set of their sculptures is being cast in bronze in readiness for a show in January at Tate Britain (Jake has hurt his back and can’t be here). The foundry specialises in producing artists’ work. Hanging from pulleys are two huge, bronze rabbits by Barry Flanagan, while several sections of a gleaming Anish Kapoor await a final polish.

Looking around, it is apparent that the Chapmans haven’t gone nice: their work still heavily features sex, death and not a little violence. A club bristling with nails prepares to snuff out a candle stuck into a brain, while beneath it maggots wriggle among disintegrating grey matter and a severed penis. In another piece, a phallus attached to a long hose advances suggestively towards the neck of a milk bottle, as an inflated glove gesticulates comically overhead.

The Chapmans, clearly, are still out to shock, but can they still cut it? They first found notoriety by recasting Goya’s Disasters of War in threedimensional form, which they showed at Saatchi’s seminal YBA jamboree, Sensation, in 1997. (Six years later they returned to the work, drawing clown and puppy faces on to Goya’s original.)

Also at Sensation they exhibited Zygotic Acceleration, Biogenetic, De-Sublimated Libidinal Model (Enlarged x 1000), a ring of dolls melded together, with penises for noses and other hideous deformities. Hell (1999) was their most extreme sculpture; the nine tableaux were populated by thousands of tiny soldiers and mutants carrying out acts of torture typical of Nazi concentration camps.

When the work was destroyed (in the Momart fire in 2004 which annihilated a number of YBA works), some saw it as an act of God. Yet the brothers refuse to repent; on the contrary they continue to explore the theme in sculptures such as Arbeit McFries, a horrible pun on the Arbeit macht frei sign over the gates of Auschwitz. In Sex, exhibited when they were shortlisted for the Turner Prize in 2003, skeletal body parts covered in maggots and worms hung from a dead tree; hand-painted with gory hyper-realism, the grizzly scene is so over-the-top that it is ridiculous rather than horrific.

I once gave a talk called The Schlock of the New in which I claimed that the Chapmans, as artists, were working themselves into a corner. If you are hell-bent on breaking taboos, you are bound to run out of material that shocks and, by defining your territory in opposition, the work will inevitably be parasitic. But I underestimated their Houdini-like ability to wriggle out of tight corners and find fresh pastures.

Last month, for instance, they took up portrait painting. They set up shop at the Frieze art fair and worked all day in a makeshift studio. It was a veritable production line; each session lasted 30 minutes. Although we had paid to be drawn we had little say in the outcome. Rumours circulated that a major collector had been portrayed as a steaming turd.

We were allowed one request, so I asked Jake to be unusually kind. Jake muttered that he couldn’t “find” me. Lining the walls were portraits of people as spiders, dogs, owls and a trademark F*** Face, endowed with a penis for a nose and an orifice for a mouth.

“I wouldn’t like to be a F*** Face,” I remarked helpfully, and immediately a gleam appeared in Jake’s eye as he asked me to smile. The portrait is a remarkably good likeness but, you’ve guessed it — I’ve got a long, phallic nose, and gossamer wings. At least I’m smiling.

Over five days, the brothers produced 127 portraits — a mind-boggling 15 paintings a day. The portraits will be on show in Liverpool as an installation, hung on the walls of the temporary studio. The title Painting for Pleasure and Profit: A piece of site-specific performance-based body art in oil, canvas and wood, makes it clear that the quality of individual portraits is not an issue; this is a demonstration of bravado, a flaunting of rapidly-acquired skills to debunk the mystique of portraiture in particular, and oil painting in general.