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Making things look rusty has always been a goal of mine. Whether they're rusty tin cans or something that only looks rusty, this just adds to the mystique.
Grunge is simple to make, and almost idiot proof to apply, if you don't mind a bit of mess. The mix itself is dry coffee grounds mixed with various spices like cinnamon, nutmeg or allspice, whatever you have handy in large amounts.
I buy the biggest and cheapest ground cinnamon from the local bargain shop to use for this, and save and dry once used coffee grounds.
Although other spices like chili powder are the right color, unless you really like the smell of it I wouldn't use that. Stick with the apple pie flavors!
Mix the dry ingredients together and keep then in a jar with a lid, to keep the scent in, and to keep it dry.
When you're ready to use it, put some into a bag, and then spread Mod Podge or other white glue on your item, dunk it in the bag and shake it around a bit to cover. Then let it dry.
You'll find out where you didn't apply the glue enough by the bare spots - no problem. Put more glue on those, dunk again until it's all covered. You can also put Mod Podge all over the item to seal it and stop the 'grunge' from rubbing off.