The artist known as Eric Johnson is the spit of a youngish Oliver Reed. Ruggedly handsome and dressed in black, his rolled-up shirt sleeves reveal large tattoos. A huge panther crawls down his right arm while, on his left, there's a portrait of Lucky, a dog he once owned.

He is surrounded by 40 huge canvases. Each is in his trademark graphic style: a bold wash of colour with a single, arresting image. A family with blacked out faces. A pig dressed as a man. One just shows a huge N, another the letter P. Brightly coloured bottles of acrylic paints and brushes overrun a small trunk on a paint-spattered table.

But Johnson's is no ordinary studio. We are on the stage at the Hammersmith Apollo, it's 7pm and Neil Young fans are trickling in. They've come to see the legend during his ten-day stint in London.

“It was Neil's idea,” says Johnson, 40, who paints throughout the 3-hour show. Working on up to 11 paintings a night on an easel at the back of the stage, he also “delivers” paintings to an easel at the front, each of which refers to a different song. The P painting refers to Pegi Young, and sits there while Neil's wife opens the show. The N? You guess.

Johnson has 50 canvases with him on the tour, alongside the four silver coaches, the stage set, the rig and the more than 50 instruments including a psychedelic baby grand piano that it is Johnson's day job as road manager to oversee.

“Yes, it is a lot to do, but I love my job. It's a thrill to work for Neil. Now he's asked me to paint every night; the thing I am most passionate about in my life.”

Johnson grew up in Seattle, Washington. His mother was a travel agent, his father a teacher and political lobbyist, whose hobbies included pyrography: “One of my strongest memories is the burnt wood smell around our house.”

While managing his college's radio station he fell into the music industry. It was the late Eighties and Seattle was the music centre of the world. Johnson booked Soundgarden to play at his college and they became friends. He made them some band T-shirts, they loved them, and asked him to do their merchandise while they toured EuropeHe has never looked back. The first T-shirt he made is now in the Rock'n'roll Hall of Fame in Cleveland.

It was while working with Pearl Jam in 1994 that he met Young. “They were touring together. There were 36 people in the band party, 11 kids, three aeroplanes and no luggage guy or assistants. Neil could see that I loved my work and that I didn't ever give up. I guess he thought it'd be fun to steal me.”

Young obviously approves of the diversity of Johnson's talents. He cast him as the Devil in his bombastic eco musical, Greendale. “That was wild. As well as the film, we did 80 stage shows. I even had dance numbers. One of Neil's many gifts is realising people's talents - letting other people shine.”

Johnson has always made art. “I just like to make things. Doing what I do means having to stick around your hotel a lot. I spend that time drawing and painting.” He sleeps only four hours a night - “I might be missing out on something.”

His favourite works are what he calls his “guerilla installations”. While working with Young on the album Prairie Wind in Nashville in 2005, he became a local hero when he doctored an unpopular public sculpture. “In the middle of Music Row there is this huge statue called La Musica,” Johnson says. “It's a huge bronze of six 15ft naked figures, and people don't like it. They don't understand what it has to with music.”

So he bought plywood and paint and made a 10ft guitar, banjo, bass, fiddle and several huge belt buckles. He then attached them the statue with steel cables. “The locals loved it,” Johnson smiles.

A road manager for 20 years, he says he keeps doing it “so that I can afford to be an artist”. However, unlike most struggling artists Johnson now has an audience of 3,000 every night. He's aware of his double-edged position. Most people won't take any notice of him, and some rock critics have been less than complimentary about the pictures. “If people like the paintings, that's a bonus,” he smiles. “But if they don't, you only have to look to the left and that's the real entertainment.”

To see Eric Johnson's paintings go to www.neilyoung.com. Neil Young plays Hammersmith Apollo, W6 (0844 8444748), tonight and tomorrow

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