Brace yourself, reader. This is an account of a conversation with the director of the yucky trailer-trash comedy Pink Flamingos. Perhaps you won’t recall the final scene in which the overweight transvestite Divine munches on an actual dog turd. No, it wasn’t faked — this was in 1972 and there was no budget for trickery.

‘Because we were on pot all the time it didn’t seem that strange,’ John Waters recalls. ‘It’s lost today, but it was a political commentary. At the time Deep Throat had just come out; pornography had become legal. What’s left? What can’t you do?’

Waters is celebrated for his pencil moustache and transgressive movies, which shake a (knowingly limp-wristed) fist at the tyranny of good taste. When the Sixties celebrated the heterosexual revolution, his early work went several extra miles further to explore the boundaries of what could be put on film. His exuberant comedies tell of fat girls who wanna dance (Hairspray), suburban murderesses (Serial Mom) and sex addicts (Polyester, A Dirty Shame). And those are just the polite ones you can take home and introduce to Mum and Dad that were made after the film industry suffered him to enter the mainstream.

As a body of work, his filmography represents a phantasmagoric challenge to American moral orthodoxy, and as the culture of free speech gets wrapped up in a new set of rules and regs, Waters’s films still have enough pep to nip at your ankles. He predates Jeff Koons’s porno selfies and Hollywood’s bratty tittering over jism gel. And for that reason alone Waters’s crusade against calcified values merits celebration: he is an all-American pioneer, whose frontier was the very wild west. The whole canon will come out to play again in a splashy retrospective at the BFI Southbank in September, which includes Pink Flamingos, only recently unbanned in the UK. ‘A great moment with no irony,’ he says.

Waters hasn’t actually made a movie in a decade, US independent cinema having collapsed on him. There have been two stage musicals of his films — his comedy about juvenile delinquents, Cry-Baby, flopped, while the stage Hairspray continues to outstrip his films commercially. Meanwhile he’s written the odd book — one about hitching across America (‘I needed to scare myself a little’). And he has continued to dabble in visual art, the latest products of which are taking a bow in snooty old Mayfair.

We meet in the gallery, where Waters himself looks like an exhibit, immaculate and whippet-thin in an aquamarine blazer and in need of none of the lurid nips and tucks he imagines for himself on one of the canvases. Waters is 69, a good age for someone who likes to have it both ways: to shock while wearing an imperishable air of schoolboy innocence. Take his position on pubic politics, which reeks of both ornery good sense and flamboyance.

‘Young people don’t have crabs,’ he advises, ‘because they don’t have body hair, which I find alarming. I went to court so you could see bush and now there isn’t any!’

Then there’s transgender. While the rest of the world ties itself in knots in order not to fall foul of PC/LGBT lexical updates, Waters happily alludes to Vanity Fair’s post-op cover star as Bruce instead of Caitlyn. ‘The Bruce Jenner/Caitlyn thing is kind of old hat to me. I had a transgender woman in Pink Flamingos. She had breasts and a penis. She got the operation that week. That was radical to me then. Caitlyn’s a Republican, she’s on a reality show, and she’s a Kardashian. We can’t make fun of him or her?’ He’s much more intrigued by girls who are now boys, he says. ‘It seems newer to me. They look like guys I like, so it’s even more confusing. They ask me to sign their vasectomy scars a lot, which I do.’

The art is all in this territory, a series of two-fingered salutes to the repressed Fifties in which he grew up. Classic paperback jackets are mounted alongside their porno knock-offs: ‘Rapes of Wrath’. ‘Some Like It Hard’, ‘Clitty Clitty Bang Bang’. On that fateful day in Dallas the Kennedys are followed out of the airplane by Bergman’s grinning grim reaper. The Three Stooges submit to anal surgery. Justin Bieber features as a commercially available blow-up sex doll (‘now with two love holes!’ yodels the packaging).

Waters was against censorship before he even knew what it was. As a child he refused to stand up in church and take the Catholic pledge of the National Legion of Decency. ‘It just galled me. I knew something was wrong. And the nuns in Sunday school were so evil and so fascistic.’ In Baltimore he was thrown out of every school he went to, then lasted no time at all at NYU. Does he love to shock? ‘No. I take pleasure in surprising. Shocking is too easy.’

Of course deep inside the high priest of American deviancy is a conventional East Coast liberal. He has nothing outrageous to say on Obamacare, gun laws or racism (‘Sentence them to travel!’). As for gay marriage, he’s all in favour, though isn’t tempted himself. ‘When I was young there were bars called the Hungry Hole and in those same neighbourhoods are now gay people pushing baby carriages.’ Does he fancy fatherhood? ‘I’m a good uncle — I’ll get you an abortion, I’ll get you out of jail, I’ll take you to rehab. I’d be a bad father. I’m too self-involved.’

At the same time he’s a scourge of political correctness that oversteps the mark. ‘You know you can’t say “snigger” any more in America?’

From the exhibition, you’d barely know that Waters is/was a film director. The most overt allusion to the day job is an eight-and-a-half-foot ruler, a jumbo homage to Fellini’s masterpiece (‘I used to see it on LSD, which made it even better’). There’s also a filmed retake of Pink Flamingos, but the twist is that kids are reading the script and all the transgression has been expunged.

‘I became my censor. I reshot the whole thing and there’s not one dirty word in it. It was the only radical thing I could think of doing.’