FLAT FINISHING BOXES apply mud over taped joints on walls and ceilings. Our boxes are made of superior-quality, long-wearing alloys and will last a long time. Use the 7” size for your bed coat, 10” box for your finishing coat, and 12” box for the skim coat. A single, interchangeable handle can be used with all three boxes (also interchangeable with all the major brands). Fill the boxes with the loading pump and filler adapter to minimize time and mess.