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We use a pyramid-tipped, acetylene-fueled soldering iron for our roof and gutter work; propane models and other types of tips are available. Regardless of the iron or tip used, soldering requires some prep work. Make sure that the tip is clean and that the four facets are flat and meet at a sharp point; otherwise it will be difficult to control the flow and direction of the solder. We use a grinder with a metal wheel to flatten the facets, then we sand them using an orbital sander with 80-to-120-grit paper.
Light the iron and keep it on a low setting so it heats up slowly. When it's hot enough, "tin" the tip on a scrap piece of copper. Spread flux, then melt enough solder to spread evenly on all four facets of the tip. Tinning makes for a smooth flow of solder when working a seam.
Most of the seams in a gutter are flat—no fold, just metal overlapping metal. To begin soldering, brush flux on the seam, making sure it flows between the sheets, then "tack" the seam in several places with a spot of solder. This keeps the two pieces of copper in close contact with each other so that they will heat up evenly.
Soldering is two-handed work: One hand works the iron, the other controls the solder stick. While holding the iron at a shallow angle with the tip against the metal, press the solder stick against one facet of the tip until a drop of melted solder forms, then use the iron to spread the drop across the seam. Each drop adds another layer, and the heat of the iron on the copper draws the solder into the seam to make a water-tight seal. The thick "ribs" reinforce and strengthen the seam.
If the copper gets too hot, the solder will become runny and difficult to work, especially on a vertical seam. A slightly cooler tip will give you better control as you build layers of solder. If the copper gets too hot, lift the tip off the metal for a few seconds; adding flux will also cool the seam.
