Solders and flux containing not more than 0.2 percent lead, and pipes and pipe fittings containing not more than a weighted average of 0.25 percent lead when used with respect to the wetted surfaces of pipes, pipe fittings, plumbing fittings, and fixtures shall be used in any plumbing installation which conveys a potable water supply. A Minnesota seller of lead solder, except for a seller whose primary business is contracting in plumbing, heating, and air conditioning, shall not sell any solder containing 0.2 percent lead unless the seller displays a sign which states,