Hear hear.

Stains are a function of the rain wetting and runoff pattern on the building, compounded and influenced by the manner in which the wind blows around the building.

Atmospheric deposition of solids (dust and other fun things) is washed down by rain events.  The only way to change staining patterns is to change the profiles.  

I have a fascinating case study in Beijing where a modern (late 1980s) building designed by a Western architect is supposed to draw on Chinese traditional design but in a very modernist way.  As a result we have lots of screen walls, many with openings.  All of the parapets and openings have flat surfaces with no projections.  Beijing is one of the world's most heavily polluted cities, with substantial atmospheric deposition due to a combination of ongoing low quality coal briquette burning during winter for heating and all year for cooking, and rapidly increasing automobile usage.

Combine this with a rainfall pattern which sees little rain (and typically low intensity) for 8-9 months of the year (winter) and more substantially and heavier falls for the other 3-4 months of the year.

Result is some serious rundown staining on all of the walls.  In trying to upgrade the appearance of the building, as well as to try and ensure the future longevity of the rendered finish (which is cracking along these exposed parapets), we are about to install metal cappings to all of the parapets.  These will project, in a similar manner to the traditional tiled cappings to Chinese walls, so as to shed water away from the surface of the walls.

But back to the point ... the stains will come back, albeit perhaps at a slower rate if the original cause was coal burning byproducts.  If the stains are moisture related, then it is paramount to prevent moisture entry ... but can this be done in an acceptable way?  In some cases, it cannot.  But I'm a great fan of lead weatherings on horizontal surfaces of cornices and parapets ... just that some architecture just will not accept it (crenellated parapets are a particularly troublesome issue!).

So cleaning ... I agree that the stains are part of the evidence and history of the building ... and unless the details are changed, the chances are that they will return.  But clients do love to have a nice clean 'new-looking' building which has been 'restored' to its 'original' appearance.

What have we done!!!!

Cheers

david