A method for repairing a blowout preventer using a blowout preventer milling assembly while in an operational configuration at sea. The blowout preventer is lifted while secured to casing and the blowout preventer is locked to the platform of a rig. The opening doors of a gasket chamber of a blowout preventer have a gasket surface for resurfacing. A pair of legs slides into grooves in the gasket chamber. A support frame is attached to the pair of legs using a plurality of spring loaded mounting assemblies. A pneumatic spindle with a calibration head inserts into a spindle holder of the support frame. Each actuator of the support frame is connected to a portable controller. The portable controller calibrates a location of the pneumatic spindle over the gasket surface for resurfacing using the portable controller.