I have used coarse sanding drums and/or the little wire brush wheels run at med-low speeds in a Dremel, quick and easy, I even got the glue to hold on some factory pre-finished leather. When they tell you that these pre-finished leathers can be worked with just like like veg-tanned, walk away very quickly.
PS: The larger wire wheels, run in a hand drill work great also for larger jobs. Start slow until you get the feel for it.