The Sterling Group has sold American Bath Group to Lone Star Funds, a source said. The deal, which closed Sept. 30, is estimated at $750 million, the person said. Savannah, Tennessee-based American Bath designs and makes fiberglass reinforced plastic, sheet molded compound and acrylic bathtubs and showers. Brands include Praxis, Bathcraft, Aquatic, Bootz and Clarion. The Sterling Group is making more than 4x its money with the deal, the source said. Executives for Sterling declined comment. Lone Star and American Bath couldn’t be reached for comment.