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Body Solder Application Guidelines

Traditional lead-based body solder has been the choice of restorers and customizers for over 80 years for filling seams, leveling uneven body work, and blending-in custom features. Even the best polyester body fillers available today cannot match the superior adhesion, strength and overall durability that body solder provides. The following article will compare the benefits of lead-based body solder vs. lead-free body solder, and provide a step-by-step application guide for applying body solder.

Leaded vs. Lead-Free Body Solder
Traditional leaded body solder bars consist of 30% tin and 70% lead. This mix produces a solder that is easily applied to vertical and horizontal surfaces with a relatively low spreadable range (361° to 489°F). Tensile strength (the amount of force required to pull a substance apart) of leaded solder is 6,140 psi.

Body solders containing lead MUST be leveled by filing -- sanding is never an option, because sanding lead-based body solder would put toxic lead dust in the air, and grit from the sandpaper may embed in the solder and cause corrosion.

Because of the inherent danger associated with lead-based body solder, lead-free body solder was developed as a safe alternative.

Over the years, we’ve used a variety of lead-free body solders while restoring vintage cars. All the lead-free solders we tried would turn to liquid and run off the panel before any shaping could be done.

However, Eastwood sells a lead-free formulation that is much easier to use than even leaded body solder. We were able to readily apply as much solder as we wanted to the vertical and horizontal surfaces of a fender without it running onto the floor.

Lead-free body solders available in the past would typically require much higher heat to melt and were very difficult to keep on anything other than a horizontal surface. But Eastwood’s lead-free body solder has a spreadable range of 428° to 932°F (220° to 505°C) and a tensile strength greater than 9,000 psi. The increased strength makes this lead-free solder more appropriate for building-up door and other panel edges, and style lines. Leveling can be done by filing and sanding, since there are no lead particles to be dispersed. (Be sure to wear a dust mask as you should for any metal-grinding operation.)

This Eastwood formulation is one of the few solders that can actually be powder-coated and cured at 400°F (with accurate temperature control) without deforming. The fact that the lead-free solder can withstand powder-coating temperatures is a big benefit because it provides an alternative to Lab Metal for filling irregularities in iron and steel parts that will be powder-coated. Keep in mind that most of Eastwood's powders are cured at 400°F -- this is 28 degrees less than where this solder starts to soften. The solder is weak at 400°F but will not deform.

Getting Started - How to Determine if a Panel can be Soldered?
Once all coatings have been removed from the surface to bare clean steel, heat the area with a propane or MAPP gas torch. If the panel sinks as heat is applied, it should not be soldered. The way the panel reacts to heat indicates the stresses that were imbued into the metal when it was manufactured. Exposure to heat can actually produce stress fractures in some steels, so be sure to only use a sufficient amount of heat to melt the solder.

Avoid soldering perforated panels because the flux residue on the back of the metal will cause accelerated corrosion. This problem usually shows up as a swelling in the repair area a few months or years later as the forming rust underneath expands. For this same reason, seams that are only partially welded should not be soldered. Seams should be completely welded to prevent acidic flux residue from becoming trapped.

Step-by-Step Soldering Process (For Leaded and Lead-free Body Solder)
1) First, the steel needs to be clean, bare metal, free of any coating, plating, or rust. A Nylon Cleaning Wheel like our Cleaning Disc 31095 does a gentler job of cleaning the surface than a grinder, without unnecessarily removing metal. Be sure to clean a few inches beyond where the solder will be applied. Wipe the surface with PRE Painting Prep or acetone to make sure the surface is free of grease and oil.

2) Apply the tinning compound (or flux if you are using the lead-free solder). This is typically a fairly thick mix of tin powder and zinc chloride. It usually requires a little stirring to get all of the solids evenly distributed. Once stirred, the flux is applied, slightly beyond where the solder is to be applied. The surface is then heated with a propane or MAPP gas torch until the fluxed surface takes on a silvery-brown foam look. When this happens, take a clean white cotton cloth and wipe away surplus flux. You should be left with a bright silver tinned coating.

3) Clean the tinned surface. Most of the cleaning is done by using a clean white cotton cloth dipped in hot water. This is most effective while the surface is still hot from the tinning process. A surprising amount of residue can be removed simply by rinsing with hot water, since the residue is basically a type of salt. It’s beneficial to follow this rinse by scrubbing the surface with a dilute solution of baking soda and water to neutralize any acid residues, and then thoroughly water-rinse.

4) Apply the solder. The heat from a propane or MAPP torch works fine here. Basically train the heat on the surface and the tip of the solder bar with the solder bar touching the tinned surface at about a 45 degree angle. As the tip of the solder bar starts to melt, deposit nodules of solder on the surface. Try to apply a bit more solder than what you think will be required to level the surface. It’s possible to hold a few bars together when filling large areas. It’s much easier to remove surplus solder than to try to add additional solder. Avoid the tendency to over-heat the surface, otherwise all of the solder will end up on the floor.

5) Lube the solder paddles with the tallow or lube. The lube prevents the soft solder from sticking to the paddle. Heat the solder gently until it slightly dulls and starts to look a little smoother. Immediately, gently, use the paddle to push the solder into the basic shape you need.

6) After the surface cools, use the flat flexible file to refine the shape. Even though this file is a coarse 8 teeth per inch, it leaves a smooth surface. The solder files much more quickly than the surrounding steel so check the shape frequently to prevent undercutting the solder. It’s important to remember that lead-based solder should not be sanded because it puts fine toxic lead dust in the air and imbedded abrasive can cause corrosion. Lead-free solder can be sanded with appropriate eye and respiratory protection.

7) Once the surface has been shaped to the proper contour, wash the repair area and the surfaces surrounding it with a baking soda and water solution to neutralize any residual acid from the flux operation. It’s a good idea to follow this with a wipe down using Fast-Etch Rust Remover. This will eliminate any small pits. Wipe the surface with PRE Painting Prep or Acetone and dry with a clean soft cloth.

8) At this point a skim coat of polyester body filler can be applied to get the contour exactly right.

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