Remove the membranes from the back of the rib racks and pat them dry.
In a bowl, mix together the brown sugar, garlic powder, onion powder, seasoning salt, black pepper, paprika, chili powder, cumin and thyme until a dry, crumbly mixture forms.
Rub this mixture all over both sides of the rib racks and wrap all three racks in aluminum foil, individually, placing them on a baking sheet (or two, if needed) and into the oven at 300 degrees Fahrenheit.
Bake the ribs for about 1 and a half hours, fully wrapped in foil.
Remove the ribs from the oven and open the foil, draining any liquid. Place the racks of ribs on the baking sheet (line it with more foil, if desired) and brush a thick, generous coat of barbecue sauce on all sides.