Prepare the smoker for indirect cooking with very low heat (225° to 250°F).
Mix the rub ingredients. Put the spareribs, meaty side up, on a cutting board. Follow the line of fat that separates the meaty ribs from the much tougher tips at the base of each rack, and cut off the tips. Turn each rack over. Cut off the flap of meat attached in the center of each rack. Also cut off the flap of meat that hangs below the shorter end of the rack. Using a dull dinner knife, slide the tip under the membrane covering the back of each rack of ribs. Lift and loosen the membrane until it breaks, then grab a corner of it with a paper towel and pull it off. Season the racks all over with the rub, putting more of the rub on the meaty side than on the bone side.
Fill a small spray bottle with 1 cup of water.
Add the wood chunks to the charcoal. Smoke the spareribs, bone side down, over indirect very low heat, with the lid closed, until the meat has shrunk back from the bones at least ½ inch, 4 to 5 hours. After the first 2 hours, spray the racks with water. Continue to spray the racks every hour or whenever the surface looks dry.
When the meat has shrunk back from the bones at least ½ inch, brush the racks on both sides with a thin layer of sauce and wrap each rack in heavy-duty aluminum foil. Return the foil-wrapped racks to the smoker, stacking them on the top cooking grate. Continue to cook, with the lid closed, until the meat is tender enough to tear with your fingers, 45 minutes to 1 hour more. Remove the spareribs from the smoker and lightly brush the racks on both sides with more sauce. Cut the racks into individual ribs. Serve warm with the remaining sauce on the side.