Getting the Lead On
Applying metal body fillers
by Harold Pace
Customizing with lead body filler was so popular in the 1950s that it gave us a new term, Lead Sled, to describe a radically customized sedan slathered and smoothed with the soft, pliable metal. Today the advent of high-quality, easily applied plastic (polyester) fillers has reduced the number of experienced lead workers to a minimum, but lead still has benefits other fillers do not. In fact, "lead" doesn't even have to be lead any more!
What It Is
Lead was the first popular body filler, used by auto factories and repair shops alike. It was melted into gaps and along seams, but if improperly applied it would crack and fall out just like the cheap plastic fillers of the 1960s. However, a good lead man was much in demand by premium body shops and customizers.
The technical term for "lead" is "body solder." For auto bodywork, it is actually a mix of lead and tin, usually a 30-percent tin/70-percent lead alloy. This is different from solder used for electronic repairs, and the two are not interchangeable. Although handling lead-based body solder is not harmful, breathing lead dust is dangerous, so hand filing (instead of power grinding) and the use of a respirator is recommended. Even better, a number of companies are now selling body solders that substitute copper and zinc for lead (still combined with tin) to form safer filler materials.
But why use lead at all? There are some instances where metal body fillers are better than plastic. Joints that are subject to stress and warping are more likely to crack plastic than metal (although lead is not a cure-all for improper engineering or assembly). And plastic easily cracks away from the thin edges of fenders or doors. Lead is also considered more appropriate for high-end repairs and restoration on collector cars.
How It's Done
Working with lead is simple, but like all crafts it requires continual practice to do it well. The basic materials can be bought in kit form or purchased separately. First, the metal area where the lead will be applied must be cleaned of all paint, corrosion, dirt and grease. A sanding disc or wire brush is used to work the metal until it is clean and bright.
Next the area must be "tinned" to provide maximum adhesion for the lead. There are various tinning compounds, but most are in paste form. The area is heated using a torch (propane or acetylene) set for a soft, cool flame (a blue flame with no yellow is best). The tinning compound is then applied with an acid brush. The compound is heated until it melts and "tins" an area slightly larger than the repair area (to allow smoothing-in later).
Now it's time to apply the lead. The torch is played over the surface to keep it warm while the lead, which is in round or square stick form, is touched to the area and the end heated as well. The solder will stick to the surface, and the stick is twisted to break it off. Lead is applied until there is enough to fill the work area. Experienced bodymen know not to overheat the panel, which can cause warping, so they stop and allow everything to cool from time to time. They use just enough heat to do the job, and work slowly.
Once the lead is on the panel, it is kept just warm enough to maintain a semi-solid (plastic-like) state. Now it's time for a new tool, a wooden paddle to shape the lead (a combination of flat and round paddles allow for easier shaping). The paddles are dipped in tallow or beeswax to keep the lead from sticking to them. While the lead is soft, it is shaped and smoothed as close as possible to the final shape. Excess lead can be removed and reused if kept clean. Taking time to get the lead smooth will save finishing time later. The area is allowed to cool naturally (do not quench with water), and residue is removed with a scrub brush and water.
Now it's time to finish out the repair. Vixen files are best for hand work (and ideal for beginners), but if you are using lead-free body solder and have lots of experience you can use a grinding wheel. Although professionals sometimes brag about not using any plastic filler, most of the time a small amount is needed to fill tiny pinholes and imperfections.
If done properly, leaded repairs or customizing should last longer than those done with plastic fillers. However, if the surface is not cleaned properly it is possible for contaminants to get trapped in the lead and attack your paint job down the road. The importance of cleanliness cannot be overstressed. As with all such projects, practice on an old body panel before you turn the torch on your pride and joy!
Cool!
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