If you stand on the 45th floor of the tallest building in downtown Sao Paulo, the chaotic concrete jungle stretches in all directions as far as the eye can see. The energy and traffic fumes produced by this Brazilian city’s 18m inhabitants create a haze that almost obscures the sky, and any evidence of green space is scarce from this vantage point.

It is here, in the city’s rundown bustling centre, that you’d usually find the Sao Paulo crew of some of Brazil’s most highly respected graffiti artists. They include Gustavo and Octavio Pandolfo, known as ‘os Gemeos’ (the twins) and their fellow crew members Nina and Nunca. For almost two decades, they have lived and breathed graffiti, gradually gathering international acclaim.

But two weeks ago they traded in Sao Paulo for something very different. Their latest month-long project has brought them to the 13th century Kelburn Castle, near Largs in Ayrshire, where they have swapped skyscrapers for rolling hills and rivers, and traffic noise for the sounds of birdsong and buzzing bees.

Their task is an unusual one - by the end of May they will have transformed the once-drab grey exterior of the castle into an explosion of colour, as they mix influences of magic and mysticism, Brazilian myths and Scottish history, into a gigantic piece of graffiti art, covering the castle and its turrets from top to bottom.

Alice and David Boyle - the children of Patrick Boyle, the 10th Earl of Glasgow - who have backgrounds in art and architecture, first had the idea when they heard the concrete rendering, which dates back to the 1950s, would need to be replaced with a limestone one, as part of a £1m refurbishment project due to start in 2009.

It was Alice who first suggested they paint something on the wall before it came down. ‘The ideas spiralled from there,’ she says. ‘We thought it would be brilliant to take an art form like graffiti and put it somewhere that was so different from its usual context.’

For most people it would probably have gone no further. But the Boyles - descendents of the Boyvilles, one of Scotland’s oldest families who settled in Kelburn in 1140 - were determined to make it happen.

Getting it off the ground has been no small feat. First they had to talk their extremely apprehensive father onboard, then convince Historic Scotland to give it the go-ahead.

Then there was the small matter of persuading Brazil’s most established and in-demand graffiti artists to clear their diaries and travel thousands of miles to rural Scotland.

Add to that the need to raise funds for the £20,000 project - much of which was gathered from cold calling local businesses - and they found themselves facing a steep learning curve.

‘There are so many highs and lows,’ says Alice. ‘We got an email a week before saying “So you’ve primed the walls?” We didn’t know anything about it and I was about to have a heart attack. Then the paint didn’t arrive on time. And then the machines didn’t get here.’

‘But it’s never been a worry that the work wouldn’t come up to scratch. I’ve never seen anything that they’ve done that I haven’t liked.’

Her positivity is infectious. David agrees, ‘I think general excitement kept us going,’ he said. ‘Gradually we started to realise that everything we were doing was going to pay off. Now we can sit back a bit and enjoy the project.’

So far this afternoon ‘sitting back’ has involved answering phones that are ringing off the hook, arranging access for TV crews, updating the website, picking up posters from the printers, all in between interviews.

Amongst the flurry of activity, the artists are an oasis of calm, producing an impressive amount of work. The castle tower, which rises skyward like a totem pole is a mishmash of images drawn from indigenous culture, with a huge all-seeing eye staring out from its centre. On the next wall, a yellow-faced fairy-like creature flits amongst the outlines of butterflies.

During a well-earned break they laugh as they acknowledge how different their new working environment is. ‘Arriving here was a big shock,’ admits Nunca. ‘We don’t have castles in Sao Paulo.’ Now everything they do revolves around one. The crew will work, eat and sleep here for the duration of the project.

‘Brazil’s history is just 500 years old,’ says Gustavo. ‘The wall we are painting dates back 800 years. It’s amazing to think about that as we work.

‘Sao Paulo is totally urban, chaotic, without laws or rules.’ He looks around. ‘It’s totally the opposite of this place.’

All are immersed in South American street culture. The twins got involved with the hip hop scene in the late 80s, while Nunca was out on the streets, running with a group of taggers at the tender age of 12.

Nina, a trained painter, says, ‘Graffiti art is there on the street for everyone to see, without discrimination in terms of social class, or age or anything else.’ And in a city like Sao Paulo, where the gulf between rich and poor is one of the largest in the world, that’s important. ‘You can use graffiti, not only as a form of art but also as a means of protest,’ she explains.

Os Gemeos, whose strangely appealing bandy-legged yellow-faced characters can be seen staring down from buildings across the South American city, claim they are trying to offer an alternative world from the harsh realities of life in Sao Paulo.

But they are also influenced by their immediate surroundings. Here that is felt in their sense of the history of the wall they are painting on. As they lie in bed at night, Nina explains, they think about the people stretching back through the centuries who once lived there.

It’s time to get back to work. Gustavo powers up the cherry picker, Nunca takes up the paints, while Nina and Octavio sketch out another section.

The Earl stops by to check on progress, and beams, his nerves gradually soothed by the evident talent and charm of the artists involved.

He is hoping it will attract a new audience to the castle. ‘What is great about this project is that it is a real clash of cultures,’ says the Earl. ‘What could be more different than an ancient Scottish castle and South American street culture? But somehow it works.’

Nunca adds: ‘This project is not just about Brazil, about Scotland, or about painting the walls of a castle. It’s about different points of view. It’s about seeing things from a different perspective.’

You can see the artists at work at Kelburn Castle and Country Centre, near Largs in Ayrshire, until 9 June. For more information and regular updates on the project go to www.thegraffitiproject.net