It depends on the setup, there's more than one way to do it but probably you want to go the Shading tab of the Sketch Style Tag and check 'Enable' there but set 'Object' to Off. That allows you to use other textures to define the shading but keep the lines from the Sketch material. A quick example using two textures, one for color and one for cracks alpha'd on top - but it can be done with one texture and different set ups as well. It's set up assuming I understand your description, grey with cracks and the cracks disappear as it goes to white and with noise instead of a psd for cracks...
334_sketchtest.zip
