The show of some 70 paintings by McCartney, dating from 1987 to 2001, opens Friday at the Walker Art Gallery and runs through Aug. 1 in the singer-songwriter's hometown.
"I used to come here as a schoolboy," McCartney said. "If I had said to John then, 'I'm going to have an exhibition here one day,' I think I know what he would have said. I'll leave it to your imagination."
Casually dressed in a T-shirt, denim jacket, black trousers and sandals, McCartney, who turns 60 next month, looked relaxed and fit. He chatted affably as a media throng stood, kneeled and squatted before him at a preview Thursday of the exhibition.
McCartney's brightly colored and slapdash paintings are shown with six sculptures made from driftwood. None of the works, taken from his homes in Britain and the United States, are up for sale.
His canvases depict landscapes, shells, flowers, people with often wild faces and Celtic mythology - McCartney has Irish roots. Most of the pictures are sunny but several are dark and mysterious. One, "Bowie Spewing," shows fellow pop icon David Bowie being sick.
However, McCartney did not claim to be a great painter. "I'm not out to show the world what I can do. I'm not trying to impress anybody except myself. I think I've shown the world enough already," he said.
He said he wanted visitors to the gallery to enjoy the images, the colors and the freedom of his paintings.
As for the critics, McCartney said he wasn't very interested in their opinions.
"Some will like them and some won't. They're entitled to their opinions but I never read them," he said. "I really just do it for my own enjoyment. There's so much to learn, that's half the fun."
Julian Treuherz, keeper of the Walker, said the gallery was planning to limit the large crowds expected.
The Walker opened in 1877 and is filled with the works of such old masters as Rembrandt van Rijn and Nicholas Poussin, and the pre-Raphaelites. The gallery has a long tradition of displaying local artists' work.
McCartney retains a house in Liverpool, which he visits regularly; his composition, "Oratorio," has been performed in Liverpool Cathedral.
His art was first seen in 1999 in Siegen, Germany, where a local gallery persuaded him to do a show and the enthusiastic response let him know there was an interested public.
Michael Simpson, the former curator of modern art at the Walker who selected the paintings on display, said McCartney's work was all about heart.
"Love and passion are resonant themes in his music and so, too, now in his paintings. Without heart, art is not art, it is meaningless," said Simpson.
Among the abstract paintings is a large red heart with the figure of a female nude scratched on the surface titled "Big Heart, 1999." It is one of the few works in the show painted by McCartney since meeting his fiancee, Heather Mills, after the death of his wife, Linda.
"Big hearts is what Paul and Linda said they most wanted for their children, and a big heart is now what he plainly has for Heather," said Joe Cooper, the Walker's communications manager.
"The Kiss," shows McCartney kissing Linda and "Hottest Linda," depicts his late wife sunbathing.
McCartney won an art prize for a drawing of a church when he was 11 but he only began painting seriously 20 years ago. Since then, he has produced some 600 pieces.
"For me it's an enjoyment thing," he told reporters. "I basically like to apply paint onto canvas. I just like the act of it -- I make it up as I go along."
The exhibition will only be shown in Liverpool, where all four Beatles were born, grew up and came together as a band.