Although leading joins is an old and traditional method of assuring that a join is properly sealed and doesn’t corrode, it is still the best way to finish after two panels have been welded. After the two panels have been welded and the join ground flat, the surface is tinned, lead will only stick to lead so tinning a panel burns small particles of lead in to the metal and removes all impurities, this assures a good adhesion with the two metals.
Once the lead is in to the join wooden tools known as puddling sticks can be used to move and shape the lead. At this point the lead must be kept warn but not too hot, if you over heat the lead it will simply turn to a liquid and fall out.
A skilled way to complete this task would be to lead finish. The lead is filed and shaped by hand to achieve a perfect shape so you can’t distinguish between the two panels, joining them as one, however this is a very long and time consuming process. Those of us that still use lead have a tendency to file as close as and then apply modern day fillers to complete the shape.