Sherwin-Williams Co. said it will buy Thompson Minwax Holding Corp. from Forstmann Little & Co. for $830 million in cash, which would give the largest maker of paints such as Dutch Boy the lead in household stains and sealers. The addition of Thompson water sealers, Minwax stains and Red Devil and Formby's products would fill a weak spot in Sherwin-Williams' lineup that includes Krylon, Ralph Lauren and Pratt & Lambert paints and varnishes. The purchase is among the largest of the 21 companies Sherwin-Williams has bought since 1990 as it expanded into related products. Shares in Cleveland-based Sherwin-Williams fell 87.5 cents to $55.875 on the New York Stock Exchange. The planned sale also represents a big payday for Forstmann Little, the closely held New York investment firm.