Clean the aluminum AC pipe with steel wool, rubbing away any grime and dirt. Scrub the aluminum AC pipe with the steel wool until the bare metal shows through.
Put on welding goggles or a welding mask and gloves. Unroll part of a coil of aluminum flux weld and set near the AC line.
Ignite an acetylene or propane torch. Hold the torch about 4 to 6 inches away from the aluminum AC pipe for approximately 7 seconds to heat the pipe up.
Retrieve the partially unrolled aluminum flux weld coil and hold it just above the AC pipe under the flame of the torch.
Let the aluminum flux weld drip onto the damaged aluminum AC pipe, filling the hole or crack. Remove the torch and flux once it's sufficiently covered and allow it to dry, harden and cool for 10 minutes.
Buff the patch on the aluminum AC pipe with a microfiber cloth once it's completely cool to finish.