Caulking Irons, also known as Making Irons, are used for packing or putting the Cotton or Oakum into the seam between the planks. Typically, Oakum is used to fill wider gaps in planking seams, whereas cotton caulking squeezes into tighter seams. It also plays a structural role in tightening up the hull / deck of a ship and reduces the longitudinal movement of neighboring planks. Extreme care must be taken as heavy hands can do considerable damage. Caulking irons are made of cast malleable iron.
For boat seams, caulking process is commonly followed by painting (seams only) with a marine paint. Once dry, the remaining space is filled with seam compound (one for above waterline, another for below) before applying overall boat paint.