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 a 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 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 eventually 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 before the tinning process (like those in a welded seam) bwill trap flux. The rear-fender-to-door-jam and side-rocker-to-fender seams were overlapped, spot-welded 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. In any case, lead as filler should be considered a substitute or shortcut for not repairing the body properly. Because of the method of body construction on the 356, lead must be used on the leading edge of the doors, and the rear door jam, but for best results its use should be kept to a minimum.
Polyester based filler has been around since the 1960s and is typically used to smooth large areas of body damage. It is made up of polyester resin mixed with filler like talc or some form of plastic powder, and can even contain aluminum powder. It can be a real timesaver, and rather than doing the metal-finishing, or panel replacement that is required to correctly repair a damaged area, body filler can be used to save time and money.
At WR we don’t use body filler and this sometimes results in very expensive repairs. That said, there is a place for this type of filler in 356 repairs when the customer simply can’t afford to have the work done correctly. We have used filler in the past on some jobs with very good results but care must be taken to keep the coats thin and to prepare the metal properly, first by cleaning with phosphoric acid and solvent, and then by blasting any pin-holes or low areas with glass bead or garnet. Polyester filler will react and swell when it gets hot (like in the sun), for this reason it’s a bad idea to fill weld seams. Take the time to properly weld, and then file the seam flat and smooth. This will provide a much better job.
Primer surfacer or blocking primer will be necessary after the metalwork is completed in order to fill very small imperfections and low areas. This type of primer is mostly resin but contains a small amount of the same type of filling media (talc, etc) used in body fillers. The factory used a synthetic enamel primer that was baked at high temperature. In the old days repair shops used air dry enamel or lacquer based primers. In the late 70’s acrylic enamel and acrylic urethane primers were developed to meet the new VOC regulations. At the same time polyester based primers also became popular. No matter what type of filling primer you use, it should be applied with a minimum of build, and the manufacturers instructions should be followed.
It’s your decision as to how you want to repair the body on your 356, and you will have to live with the results. The long-term durability of the metalwork and paint depend largely on the shortcuts taken. Lots of filler, whether lead or polyester, are shortcuts, and both have their own associated problems. My advice is to not cut costs on the metalwork. Have it done correctly and it will pay off in the long run.