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How can I avoid dark spots when applying external render?

I am rendering the exterior of my house using render to which a buff colourant has been added. When I was finishing off some dark spots have appeared on the surface (one looks like tar). Does anyone know what could cause this and how it could be avoided? The render was put on clean bricks.
Little Ed, September 2009
Me again - the render has actually been put over a clean scratch coat and not bricks. Ta.

Little Ed, September 2009