Though lightweight and nonmetallic, aluminum presents a few problems when bonded. While glue provides the most reliable fusion, it won’t stand up to heavy use and breaks down much faster than soldered joints. However, aluminum oxide builds up quickly and is invisible on most aluminum sheets, making it difficult to solder. Aluminum isn’t impossible to bond, but you must work carefully and slowly, especially if you have a lot of joints to seal.
Sand the edges of the aluminum sheets with fine-grade sandpaper until they’re silver and shiny. Sanding removes the aluminum oxide that prevents aluminum from bonding properly.
Paint the edges of the aluminum with amine-based flux, an organic solder and heat-conducting liquid that melts soldering ribbon at about 140 degrees Fahrenheit. A nylon paintbrush won’t soak up the flux and ruin the brush.
Push the aluminum seams together so fluxed edge touches fluxed edge.
Unroll a little soldering ribbon and place the tip of the ribbon against the crevice between the fluxed edges. Place the soldering stylus against the tip of the soldering ribbon.
Slowly pull the ribbon and the stylus over the edges of the aluminum. The solder should flow along the fluxed edges, sealing the seam between the aluminum sheets. Unroll more soldering ribbon as needed.
Allow the solder to cool and harden for about 24 hours before putting any pressure on the seam.
Wear heavy gloves to avoid burning yourself with the solder.