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.
If the seven lessons have been gone through several times it would be expected that a side seam would be in order. It will be best to start with two pieces of galvanized iron. Make the folds and seam them as was done in Lesson 2. Instead of laying the seam on a flat surface make a bend in one of the pieces so it can be nailed to a piece of board or the bench. This bend is made parallel to the seam as shown in Figure 9. For a seam as shown in Figure 9-B where the copper has to be held up under the fold it is best to tin the copper on one side only. For a seam as shown in Figure 9-A the copper is best tinned on all four sides. To tin only one side of the copper it will be best to wipe all excess solder off with a brush or cloth and clean the copper by filing. If there is one side well tinned file only three sides and the tinning will not have to be done. If there is no well tinned side file all four sides. After the copper has been cleaned heat and, as explained in the first method under "Tinning Copper" tin only one side with rosin core solder. This is best done with the side to be tinned facing down so none of the rosin will spread to the neighboring sides. After tinning when the copper is hot enough for use there is little danger of the other sides tinning as the coat of oxide will be too heavy for the flux to remove it. The purpose for tinning only one side was explained under "Reason for Tinning Copper." The seam shown in Figure 9-A will be very easy to run as the flow of solder will be down into the fold. With the seam in position as shown in Figure 9-B it will be different.
The solder which is carried on the tinned side of the copper will have to run or sweat up into the seam. It will require patience to do this. By feeding solder to the copper in such a way that it will have to close the gap between the copper and the seam it will cause the solder to fill up the seam. The edge of the copper is placed about 1/16" below the fold to allow the solder to make contact. After a little practice this will be easy.