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R
I found the problem: menu SETTINGS>PREFERENCIES and there OPEN_GL and there choose setting with SHADOWS YES. http://sketchucation.com/forums/viewtopic.php?f=15&t=44020&p=392583&hilit=shadows+faces#p392583
T
It needs to have been made properly in the first place ?
As an alternative to 'flatten'... you might try to group all edges etc and then make a large flat surface below them, use the Sandbox tool and Stamp the group onto this flat surface ?