LEAD vs. POLYESTER FILLER

On our 356s, lead was used to fit the removable panels (hood, doors, etc.) to the body. At the time, this was an inexpensive way to build the bodies. Lead is an effective and permanent filler for steel bodies and is the only material that will withstand the heat associated with the baking process of synthetic enamel.

The lead used as a filler for bodywork is not pure lead, it is a combination of lead and tin, usually in a ratio of 60/40%. In order for the lead/tin alloy to stick to the metal, the metal must be cleaned with a flux. Liquid flux is applied to the metal area, as it is heated a very thin coating of lead is applied and spread onto the metal with steel wool and then wiped clean with a rag . It is important that the finished surface be shiny, clean and free of all traces of flux residue.  This process is referred to as "tinning". There are several ready- made products available for tinning which are basically flux mixed with granulated lead either in a powder or paste consistency. While they make the process easier, they are more prone to trap flux in the tinning coat which will result in a contaminated job that will produce bubbles and blisters in the paint. Also important to remember is that  pin holes, open seams or any spots that cannot be polished completely smooth (like those in a welded seam) before the tinning process begins will trap flux. The rear-fender-to-door-jam and side-rocker-to-fender seams were overlaped, spotwelded and filled with lead at the factory. These areas typically have problems and lead swelling from the trapped acid is common. They were originally done this way to speed the production process.

After the tinning process the lead can be applied. It is usually available in sticks or bars about 18 inches long that can be heated and applied to the body panel. A wooden paddle is used to spread the heated lead onto the body panel as desired. It must be kept at just the right temperature: too hot and it will run off the panel or melt the tinning coat, too cool and it won't spread evenly.  A light oil or ATF must  be used to keep the lead from sticking to the paddle. Wax is often sold for this purpose but has a tendency to stick in the tiny pores of the lead and once cooled is very difficult to remove. Again, the resulting problem is bubbles and blisters in the paint.

The use of lead as filler is a painstaking process that requires considerable time to perfect. Juggling the torch, lead and paddle, especially on a vertical panel, can be a challenge. Added to this is the fact that heating most flat, or nearly flat, body panels hot enough to apply the lead almost always results in some warping which must be addressed by additional metal finishing and filing.

Polyester based fillers can be used in place of lead with good results and require much less time.  We use a product called All-Metal.This  filler consists of aluminum powder mixed with polyester resin and dries much harder, and is more stable, than the typical talc based body filler and will not absorb solvent.   An extremely clean surface is mandatory before application, and all welded areas must be spot blasted and cleaned with phosphoric acid and lacquer thinner before application. 

When making the decision to use lead for repairs and panel fitting it is wise to consider the problems associated with it.  Polyester based body fillers can give excellent results for much less money.  No matter which method of repair you choose for your restoration, the metal should always be repaired so that a minimum of filler is used.