This section is from the book "Soldering For Workshop, Farm And Home", by John Bonert. Also available from Amazon: Soldering For Workshop, Farm And Home - Information On Soft And Hard Soldering - Projects For The Workshop Explained And Illustrated.
Soldering a seam on galvanized iron can now be tried. Two short pieces about 6" long and 3" wide will do. These two pieces are folded and seamed the same as shown in Figure 4.
Clean the edge of the pieces where they are to be seamed. Scribe a line 1/4" from the edge and bend both pieces so they can be interlocked and then pound them down with a leather or wooden mallet. Make the seam as straight and level as possible. If a steel hammer is used don't pound too hard because the metal may fold down too sharp and show signs of cracking. It is best to use a wooden or leather mallet for forming seams of any kind. Some of the solder will be expected to sweat into the seam. This is why it is important to clean the surface before seaming. The seamed pieces can be nailed to the bench or a piece of board so they will not move when the copper is moved over the seam. Don't forget a few sheets of paper under the work to conserve the heat. Go over the seam again with a piece of sandpaper or steel wool to clean any finger marks. Apply the flux which will again be zinc chloride. Go along carefully with the brush and try to keep the flux from spreading more than 3/16" from the center on each side. This will help to keep the solder from spreading too far. Now pick up a drop of solder as before and place it at one end of the seam. Allow the copper to rest at this point until you see the solder run as thin as water would and then slowly draw the copper toward the other end of the seam. This drawing of the copper must be done slowly as the seam must be heated as you go along. Of course this one drop will not go very far. When it has been used up pick up another drop and continue from where you left off. A strip of bar solder may be held to the copper and melted as the copper is moved along instead of reaching to pick up a drop at a time. Don't feed on any more solder than is needed to make a nice clean seam without piling it up. If the copper is run along slowly and the work is well heated the solder will sweat into the fold for a little distance. Use an edge of the copper for running a seam of this kind. Don't lay a flat side down as this will have a tendency to spread the solder too far. A folded seam of this kind does not require a heavy body of solder, but the crevice must be closed up all along. If this seam should be very long the copper would be moved along slow enough to heat the metal to the temperature of the molten solder but as the heat of the copper is slowly exhausted it must be moved slower and slower and finally it will have to be heated again. If the cleaning of the metal is well done and the copper is hot enough this seam will be so easy to run, it will delight the beginner. Figure 4 shows the position of the copper when running the seam.