Body Solder Application Guidelines

By John Sloane, Eastwood Product Specialist


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, Eastwood now sells a solder which works just like lead based products, but is lead free. You can read more about it here: Lead vs. Lead Free Solder.

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. Heat can produce stress fractures in steel, so only use sufficient 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 later as rust underneath forms and expands. Seams that are only partially welded should not be soldered for the same reason, completely weld the seam to prevent acidic flux from becoming trapped inside.

Soldering Process for Leaded or 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. Lead-free solder can be sanded with appropriate eye and respiratory protection.It’s important to remember that lead-based solder should not be sanded because it puts fine toxic lead dust in the air and embedded abrasive can cause corrosion.

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.