back minus front or"burning through" background with image composed of individual brush strokes and various shapes

sample bridge.cdrI would appreciate any help with using back minus front , or similar method in order to remove an image composed of unconnected artistic media strokes and or brushes from the background. The design is for printing shirts so the tshirt color can show thru the printed area. The entire bridge area will be removed from the grey so the shirt color can show through All I have been able to accomplish is to group the strokes, I have been unable to combine or convert to one single object.  In the lower drawing, the white would be the color of a white shirt showing through