I've used a cup style wire brush on an electric drill or 4" angle grinder with pretty good results. Lightly done will keep scuffing the sidewall to a minimum. Or a hand-held wire bruch with 50/50 dish washing liquid and water mixture. It ain't fast but it will cut it.
TIP: A 50/50 mixture of water and dish washing liquid painted onto the tires with a brush, allow to dry then paint to your heart's content. Paint won't stick to the soap coating; so when the paint has dried, you can just wash off the overspray.