When soldering aluminium, first thoroughly clean the surfaces to be joined with a fine-grade emery paper. Then heat the surfaces with a blowlamp until they are hot enough to melt solder pressed against them and -wearing heat-resistant gloves-coat the joint with a special aluminium solder (obtainable from builders merchants). Gently rub the solder with wire wool to spread it all over the surface – a process known as tinning.
When both surfaces have been tinned’, press them together and reheat with a blowlamp to melt the solder again. When the joint has cooled, the surfaces will be bonded.