Where lead is concerned, being lucky is far better than being good! The trick is to tin the metal around where the lead is going to be applied with acid core solder. Then the next trick is to neutralize the acid after the fill is made to keep it from causing the paint to lift. These days, finding body lead and/or acid core solder or anything else containing lead is about as likely as finding a chicken with lips!
There are a couple of Devcon products known s "Epoxy Aluminum" and "Epoxy Steel" that are much better quality and more durable than JB Weld, and MUCH easier to apply than lead. In a place where there might be a little flexing involved, it would probably be a better choice than regular body filler. 40-something years ago we used a product from Ditzler/PPG called "Aluma-Lead" which was an aluminum powder that was mixed with some sort of liquid binder.
Jerry
