The lead needs to be clean and bright to start with. If it's fairly new lead it should be solder-able without more than a scrubbing of the joints with a brass wire brush. However, if the lead is dull and oxidized, you should scrape the lead in the area to be soldered with a nail, the blade of a lead knife or other sharp edged tool until the bright metal is revealed.
|an example of paste flux|
|Example of a tallow stick. It has the appearance of a candle, but without the wick.|
|Example of the application of tallow to a joint|
Then the flux can be applied. Paste flux or tallow works best as neither flows in its cold state. This means that you can flux the whole panel at one time without the liquid flowing away or drying. Once the whole panel is fluxed, you do not need to stop during the soldering process.
|Ecxmple of a gas powered soldering iron. The flat face of the soldering bolt is held in full contact with the joint.|
An electric soldering iron is held over-handed (as you would a bread knife) in order to get the handle low enough to have the tip flat on the lead. This will be a 15 to 20 degree elevation from the horizontal. Allow the weight of the soldering iron to do the work for you.
|Example of smooth flat solder joints.|
Avoid "painting" or dragging the iron across the joint. The object is to have a shiny, smooth, slightly rounded solder joint. Moving the iron and solder around does two things. It makes for a weak joint as the solder does not have the chance to become stable and so forms a "pasty" joint. Moving the iron around during the soldering of the joint often provides sharp points where the iron was moved quickly off the joing. There should be no points sticking up from the solder joint. If a solder joint is not satisfactory you can re-flux and re-heat. Don't apply too much solder. It's easier to add more solder than to remove excess.