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How to heat and apply solder
Cut, file, sand or otherwise flatten the joining surfaces as much as possible.
However don't overly polish the surfaces as a slightly rough or' keyed' surface usually leads to a stronger bond. It is crucially important in any soldering task to ensure that the pieces to be joined match up closely with the smallest gap possible.
Wash the pieces clean and if possible' pickle' them. Pickling means immersing the pieces or covering the joint area in an acid cleaning fluid.