The concrete effect is done in a couple of steps.
Paint the surface with plasterboard joint filler ( As builder Ive got a 10litre tub in the garage
Its pre mixed and dries light beige) The filler is diluted slightly with water to allow it to be applied with a brush. I applied it in horizontal bands to a pencil mark to try and simulate casting joints.
The finished plaster then had white enamel paint added to represent the lime staining.
Last step a thin wash of acrylic matt black to weather which is applied a bit thicker to darken some areas to represent water staining.
I cant get on photobucket at the moment as they are doing maintainace but there are lots of in progress pics about 7 pages back.
http://s179.photobucket.com/albums/w295/johntealon30/
John
RJR