A mixer assembly is disclosed having a container with a closed sidewall, a closed bottom and a rim which forms an open top. The container is adapted to contain a product, such as roofing sealant, plaster or the like, to be mixed. A cover overlies the open top while a clamp detachably secures the cover to the container rim. A tubular conduit is attached to the cover so that the opposite ends of the conduit are open to the opposite sides of the cover. A stirrer assembly having a driven shaft and an agitator secured to one end of the shaft is then positioned within the container so that the agitator is solely contained within the container while the shaft extends exteriorly through the conduit and above the cover. A rotary drive, such as a drill, is then secured to the upper end of the shaft to rotatably drive the agitator and mix the product within the container.