to do this i normally use what would be called a 'grunge' texture, you can find plenty of free ones online. there will be some free vector textures, that will be really heavy work for Ai to use, but if youre design is small you can use that to directly fill your shape, or use your shape and the vector grunge texture along with the path finder tools, to chop out your design from the grunge. you may get better results by using your ai vector design and taking it into photoshop, you'll have more control over the rough look and blotchy appearance etc. if you can't find a decent texture, you can place an image (you'll definitely find a good grunge texture as jpeg online) then you can use the image trace function to create your own vector grunge. all in all, test your design and cutting / fill results using a simple shape first to ensure you are using the right option, 'minus back' 'intersect' etc from the pathfinder palette, hen you won't be waiting ages for it to process, only to see it chopped the wrong part and left the waste behind for you.
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