£10m gift launches the rebirth of St Paul's

London's greatest symbol of faith and wartime obduracy, St Paul's Cathedral, is entering an architectural rebirth. A gift of more than £10m from the family of the banker Sir Robin Fleming has kick-started a £40m project that will clear three centuries of soot and grime to reveal again the brilliant skills of the church's designer, Sir Christopher Wren.

The job is a huge one and Sir Roger Gibbs, chairman of the St Paul's Cathedral Foundation, admitted yesterday that fund-raising was at the "wing-and-prayer" stage. The further £30m is still needed to complete renovations and repairs inside and outside the structure. "There have been a lot of encouraging indications, but we have not got things firmed up," he said.

Firmness was the last thing expected of the Belgian gunge being used to remove the grunge. The cathedral's internal patina is a rank cocktail of dirt, oil, iron salts, sulphur and carbon-rich pollution whose profound effects on the superb carving and sculpting began even before St Paul's was completed in the early 18th century. Wren ordered the Portland stone-clad interior to be painted "three times in oyle".

But that single decision condemned the cathedral, paid for by coal taxes and besmirched by coal smoke, to intractable cleaning problems. In 1873, the "revolting dirty" peeling paint was scraped off. But the linseed oil base had soaked at least 3mm into the stone; that ensured pollution continued to soak in.

The concoction that appears to have solved the problem is made in Brussels and partly developed by Belgium's equivalent of English Heritage. Arte Mundit V is a mixture of latex, ammonia and alkalis that more or less matches the alkalinity of Portland stone. It is sprayed on to a thickness of 3mm and, after two days, peeled off like a face-pack. The results are remarkable, although they have yet to be proved in the long term. But the early signs are good: trial areas of the interior, treated five years ago, have remained clean.

In the west transept, where cleaning is complete, the results are brilliant. The dun and grey coating, which has increasingly killed the play of shadow and sharp edges, is gone. The extraordinary handiwork of the original masons lives again. David Odgers, whose company, Nimbus, is making the repairs, said: "One simply feels in awe of the abilities of these sculptors."

In the south transept, the cleansing now shows the interweaving of sculpted garlands, ribbons, fruit and other decorative forms and motifs. Seen close up, from scaffolding 50ft (15m) up, the intricacy is even more startling. So, too, are rediscoveries such as a memorial to Captain Willetts Miller set into the wall. The carved typography, relating to a victory on the Nile in the late 19th century, is the first example of sans serif cut into British stone.

And all that prevents the indomitable but dowdy dowager at the top of Ludgate Hill from revealing herself as a pale and interesting beauty is £30m.