Body Solder Application Guidelines

Background
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 explain the benefits of 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 is 6,140 psi. (Tensile Strength refers to the amount of force required to pull a substance apart.)

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

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

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

Recently however Eastwood began selling a new lead-free formulation that is much easier to use than even leaded body solder. I was able to readily apply as much solder as I wanted to the vertical and horizontal surfaces of my 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 only 428° to 932°F (220° to 505°C) and a tensile strength greater than 9,000 psi. The increased strength makes the 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 no lead particles will be dispersed. (Be sure to wear a dust mask as appropriate for any metal grinding operation.)

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

Getting Started - How Can You Determine If a Panel can be Soldered?
Once all coatings have been removed from the surface to the bare clean steel, I 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 into the metal when it was manufactured. Some steels can actually stress crack when exposed to heat, so only use a sufficient amount of heat to melt the solder.

Avoid soldering perforated panels because the flux residues 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 residues from becoming entrapped.

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 item #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 grease and oil free.

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 a 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 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. It surprising how much residue comes off with hot water. The residue is basically a type of salt. It’s beneficial to follow 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. The tendency is to over-heat the surface and then all of the solder ends 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 above the repair 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.

www.eastwood.com