For soldering the edge of a large piece of metal, the problem is its thermal mass: it conducts heat away faster than a normal soldering iron (typically 25–80 W) can supply it.
Depending on the metal and the job, you have a few options:
- High-power soldering iron (150–300 W): Good for large copper, brass, or steel components where you still want soft solder.
- Soldering gun (100–260 W): Delivers a lot of heat quickly, though it’s better for intermittent use than prolonged heating.
- Propane or MAP-Pro blowtorch: Usually the best choice for very large metal pieces. Heat the metal itself rather than trying to melt the solder with the flame. Once the metal reaches soldering temperature, touch the solder to the joint and it should flow.
- Butane micro-torch: Suitable for medium-sized jobs, but may struggle with very large pieces.
A few tips:
- Clean the metal thoroughly and use the correct flux.
- If possible, preheat the whole workpiece slightly.
- Use a large chisel-tip on the iron if using an electric iron, as it transfers heat much better.