When is an artwork finished? It's a question that plagued artists long before Leonardo Da Vinci famously threw down his brushes (one imagines) and proclaimed, 'Art is never finished, only abandoned.' Adding a whole new curve ball to this torment is London-based artist Paul Stephenson, who gives his own finishing touches to 200-year-old works, a handful of which are currently on display at StolenSpace gallery, Brick Lane.

Stephenson buys 18th and 19th century works at auction from known artists (Alexander Melville, Thomas Bond Walker, William Allsworth) and sets about 'collaborating' with them, mingling old and new in a jarring way that the art world has rarely seen. He's already established a name for himself on the street scene for his novel 'sous rature' technique, where he physically erases sections of pre-existing paintings. This nervy act of removal, the artist explains, draws attention to the 'remaining void', and gives new meaning to the original. 'It's like when the Mona Lisa was stolen in 1911. More people came to look at the vacant space left on the wall than had come to look at the actual painting the previous week.'

StolenSpace gallery – one of a handful of London spaces drawing attention to the increasingly high-end market for 'urban art' – is the ideal home for Stephenson's 'Forced Collaboration', seeing as it is owned by fellow artist D*Face, one of his early inspirations. In the warehouse-style gallery, complete with bare-brick walls, the 'sous rature' technique has been given space to develop into a series of complex new works.

Stephenson's Watermark Paintings also use works bought at auction, but instead of erasing details, he paints new ones. The artist uploads an image of the original painting onto the online photo sharing service Shutterstock, which applies its familiar watermark. Then, he carefully paints this logo directly onto the original, and so 'the work physically and virtually exists simultaneously, in two different spaces'.

Similarly, his Reflection Paintings start life at auction. Stephenson paints phone or computer screen reflections onto original works, commenting on how we're increasingly viewing art through a screen. They're the kind of reflections you might pay little attention to, but when they're immortalised in oil on canvas, the effect on the artwork is undeniable. Stephenson explains, 'I started concentrating on these superimposed images, and seeing the beauty in them.' The billowing cloud-like additions to Reflection on Jane Camp, originally painted by Alexander Melville, 1877, redefine Camp's serene expression, and add a mythical aura to an otherwise face-on, austere Victorian portrait.

Stephenson has no issue with calling his work graffiti, and he's keen to differentiate it from other forms of 'spray-can' art. 'A painting made using aerosol on a blank canvas is not graffiti. The surface is what makes the difference.' By choosing such controversial surfaces to work on, some critics are bound to see it as a little brazen. To others, it's a fascinating way of communicating with a bygone artistic era, while highlighting the changes technology has had on the way we consume art. Stephenson simply thinks of his work as DJing. 'I am taking old paintings and mixing them with different images to make something new, like DJs did in the 1990s. I guess whether it is positive or negative depends on whether you're into hip-hop.'