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.
Before attempting a soldering job it will be best for the beginner to get some practice on junk material. Galvanized sheet iron would be a very good metal for this purpose. At most any tinner's or plumber's shop small strips of galvanized iron may be found. If this scrap galvanized iron has been cut from a new piece it will not have to be cleaned but if it is not new it will have to be either scraped or sanded down to bright metal. It will have to be free of any grease or oil. Don't be impatient while doing this cleaning. Take enough time to do it thoroughly because on this cleaning depends your success in tinning.
Clean several spots on the galvanized iron about one half inch by one inch. After the cleaning place the piece of galvanized iron on several layers of newspaper. This paper will act as an insulator and prevent the heat from getting to the bench. Without this paper it would be harder to heat the metal. Next, heat your soldering copper which you have already tinned as explained under "Tinning Copper." If you use a gas flame or gasoline torch when the copper has been heating for a short time hold a strip of solder to the point. If it melts very slowly wait just a little longer and roll the copper over slowly in the fire. If colors run around the untinned part of the copper it is hot enough for this practice tinning. For some work the copper can be hotter. It will be found that now the solder held against the point will melt very quickly. As explained before do not let the copper get red hot for you will have to clean and tin it over again.
If you are going to use an electric copper which you have already tinned, after turning on the current, wait until it will melt the solder quite fast when it is held against the point. Now with the brush apply just enough zinc chloride to wet the several spots you have cleaned. Lay a bar of solder on the bench and touch the tinned point of the copper to it. When a little solder melts raise the copper and you will find the copper has picked up a drop of the solder. If you should lose this drop don't be discouraged. Go back to the bar of solder and pick up another drop or if it has dropped on the bench you can pick up the same drop you lost. The point of the copper should always be wiped on a piece of cloth before it is used so it will be clean and bright. The beginner will soon become expert at picking up and holding the drop after a few tries. Don't move your copper in jerks while you carry a drop. Bring your copper slowly to one of the spots you have cleaned and fluxed and place the drop of solder on it. Let a flat side of the point rest on the surface until the place you are tinning is heated to the temperature of the molten solder. The solder will run like water and spread over the surface which has been cleaned. Slide the copper slowly from one end of the tinned spot to the other. If the copper is as hot as it should be this will cover the surface perfectly. If the place which is being tinned shows spots which did not take the tin it is because it was not perfectly clean. Do not use more than one drop of solder on each of these practice spots. If you heap too much solder on there may be some untinned spots covered by it.
The main purpose of this lesson is to acquaint the beginner with the importance of heat and cleanliness of the soldering copper and the work. If the copper is not hot enough the solder will melt very slowly and when it is lifted from the work the tinning will appear very rough. If the instructions have been carefully followed the tinned surface will be as smooth as the rest of the metal. After this tinning is thoroughly understood and mastered it will be good practice to take a discarded so-called tin can and do some tinning on it. If the can is new and the tin on it has not been removed by some cause such as long exposure to the weather or by burning in a fire then the flux to use is rosin. Rosin-core solder would be good for this or a little powdered rosin may be sprinkled on the surface. The tinning otherwise will be the same as on galvanized iron. If the tin on the can has been removed there will be nothing left but an iron can and in this case rosin would not be the proper flux. Zinc chloride would have to be used. And of course the iron would have to be cleaned down to new metal with a scraper or sandpaper. After tinning with zinc chloride wash the surface with water to remove all traces of acid which would start corroding the metal in a short time. This should be done on all work where zinc chloride is used. This soldering on an iron can will be a real test of the beginner's ability to tin a surface as iron requires great care in cleaning before it will tin. Once he masters the tinning of iron all other metals will be easy. It would be a good idea to try a new can and one which has lost its coat of tin. Continue this tinning practice until it is thoroughly understood.