Take ribs out of the refrigerator about an hour before putting them in the oven.
Preheat oven to 300°F. Combine spices in a small bowl. Place each rack of ribs on its own piece of heavy duty foil large enough to accommodate the whole rack – about three times wider than the rack – sufficient to be folded up and sealed. See the photo below. (You can use a double layer of regular foil if you don’t have heavy duty.) Rub top and bottom of each rack with the spice mixture (I find using a brush to distribute the spice rub gets a better coating, and less of the spice mixture ends up on your fingers). Wrap each rack individually, and place on a large baking sheet.
Bake ribs for 2 hours. Place barbecue sauce (recipe below, or your favorite) in a small saucepan. Unwrap ribs and pour 2-3 tablespoons juices from foil into the bowl with the barbecue sauce and stir to mix and loosen; discard other juices. Let ribs cool completely.
Heat gas or charcoal grill to high; oil the grate. Baste the meaty side of the ribs lightly with the barbecue sauce. Grill ribs, starting meaty side down, basting both sides with barbecue sauce and turning a couple of times, about 3-5 minutes. Transfer to a cutting board; cut between ribs using a very sharp knife. Transfer ribs to a platter; reheat any leftover sauce to eliminate any cross contamination. Serve ribs with extra barbecue sauce.