Lead body filler isn't all that difficult to do, but there is a definite learning curve. The metal must be ground clean, with no oil or other contamination whatsoever, and a fairly aggressive sanding disc needs to be used to give the metal some "tooth" to grip the solder. Good quality body solder will be either 50/50 lead/tin alloy, or no less than 60/40 % lead to tin. The metal needs to be "tinned" with acid core solder before applying the lead filler. Heat the metal just hot enough to melt the solder, and apply liberal dabs of acid core to the area to be filled, then quickly wipe the metal surface with a clean cotton cloth to spread a thin layer of solder to the entire area. This base coat is what the rest of the filler will bond to, so be sure the surface is completely tinned.
Now apply the lead filler by softening the stick of lead with a torch flame, and keeping the body panel warm with the flame at the same time. The lead should be butter-soft, not runny, and it should stick to the panel when it's dabbed onto the metal. This is where LOTS of practice comes in- - - -keeping the lead and the metal the proper temperature to be worked with a hardwood paddle and not hot enough to run off. The spreader paddle needs to have a very thin coat of non-detergent motor oil on it to keep the lead from sticking to the paddle. We used a shallow metal tray with an oil-soaked rag in it to lubricate the spreader paddle- - - -a little oil is all that's needed- - -don't overdo it! If another layer of lead is needed, thoroughly degrease the surface of the metal with lacquer thinner before attempting to apply more lead- - - -oily body solder will not accept another layer of filler.
After the job is smoothed and filed flat with a vixen file, wash the entire area with a strong solution of baking soda and water to neutralize the acid from the solder that was applied in the beginning. Otherwise, the acid will cause the primer and paint to lift at the edge of the solder filled area. Have fun, and do a lot of practicing on scrap metal before working on a project you intend to finish and paint!
Jerry
