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It depends on how hard you hit, the weight of the hairbrush, the material of the hairbrush, and whether you hit with a corner. You could bruise an ass in just a couple whacks if you use a heavy one and hit hard enough.
If you're worried about it, you might like to try Arnica cream (I think it also comes in tablet form). It's a old herbal remedy which reduces inflamation and bruising. You should be able to find it at your local pharmacy.
has used it many times.