Rather than sculpting the friezes have you considered creating a bas-relief effect by scribing some heavy duty aluminium foil? With a suitable scribing tool, I reckon you could get a decent effect indenting the back of some heavy duty metal foil, like the stuff you find sealing coffee cans and the like. You could pencil on your design (backwards, naturally enough) and then trace over it. Then glue the results to the building. Just an idea.