Tear off 2 large sheets of heavy duty aluminum foil and lay them shiny-side down. Place a rack of spareribs on each sheet, meaty-side up. Tear off 2 more sheets of foil and place them on top of the ribs, shiny-side up. Begin tightly folding the edges of the foil together to create a sealed packet. Just before sealing completely, divide the beer mixture evenly into each packet. Complete the seal. Place the packets side-by-side on an 11x14-inch baking sheet. Bake in the preheated oven until the ribs are very tender, 3 hours and 30 minutes to 4 hours. Carefully open each packet, and drain the drippings into a saucepan. You may only need the drippings from one packet. Set ribs aside. Simmer the drippings over medium-high heat until the sauce begins to thicken, about 5 minutes. Brush the thickened sauce over the ribs.