How to solder on cars

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A good soldering job is the mark of the good mechanic. Soldering is simple but has certain limitations and technical requirements.

First of all no soldered joint is strong. The main purpose of solder is that of filling up irregular places, smoothing rough surfaces, or making joints tight.

Perfectly clean metals are the foundation of all soldering. All paint, grease, and dirt must be removed by scraping, sanding, or filing. If the solder is covering a break in the metal, then other means than the soldering must be provided to make the job strong. For example, the metal may be welded and the weld hammered down, and then the rough surface filled with solder and filed smooth. But never rely on solder for strength.

After the metal is clean and structural strength provided, a flux must be applied to remove any oxidation. Probably the most common flux is muriatic acid in which has been dissolved as much zinc as the acid can "eat up." There are numerous objections to this flux from the angle of injury to hands and lungs of the user, from the fact that stronger joints can be made with regular commercial flux, plus the nuisance of preparing this flux and its cost which approximates that of good commercial fluxes. Possibly the greatest objection to the muriatic acid flux, however, is the fact that it dries out on the job, and frequently this results in soldered seams in which this drying out prevented the solder from "floating" into the seams. Thus the job was weakened because the solder merely slicked over some of the places.

Flux is generally applied with a dauber made of twisted wire and a tiny hunk of rag or with a regular soldering flux brush. The flux should be liberally smeared over the previously cleaned surface. Since heat accelerates all chemical action, the application of heat either with a torch or with a soldering iron (really a soldering copper) will improve the cleaning action of the flux.


With increased heat a little solder must be worked all over the surfaces which are to be soldered. This action is called tinning and provides the foundation of a really good soldering job.

Some mechanics prefer to tin the edges of sheet metal before bolting them or riveting them together in order that the soldering job will be more perfect since, obviously, the tinning job just described cannot be done as effectively after parts are united. Then it will only be possible to apply plenty of flux and heat and watch to see if the solder seems to be flowing through to the far side from which it is being applied. If it does not do so, then additional flux and heat must be applied which may cause it to flow, provided the surfaces were thoroughly clean before being brought together.

Methods of application

Solder may be applied four ways : poured from a ladle, spread with a soldering iron, caused to flow with a torch, or sprayed on from a soldering gun. It will be appreciated that solder when applied is in a liquid state and hence soldering on horizontal work is far easier for the beginner than soldering on vertical or nearly vertical surfaces. Another point—when possible, always tip the work so the solder may flow into the joint.

A basic point in good soldering is to have the work hot enough so the solder flows onto the job. "Puttying" solder into place is a poor procedure when sealing joints, etc., its only place being when building up a low spot in a panel.

Then having the work hotter than the melting-point of the solder would cause the whole accumulation of solder to fall off.

Building up dents in panels may be done with solder which is just under the pouring point and which chills quickly when worked into position. This paste-like solder mass is slicked smooth with a smooth block of wood which has been rubbed with beeswax. The sheet metal and the wood combine to chill this solder mass and thus prevent flowing off the job. Surface slicking is then achieved with a torch and wood paddle, the torch softening the surface of the solder (look out for too much heat or the whole works will fall off) and the wood paddle smoothing it.

When using soldering irons the important point is to keep the tip clean and well tinned. Heating an iron too hot will burn the solder coating from the tip. The tip should be kept filed to a good point with sharp edges. Stroking the tip across a sal ammoniac block will remove scale, and then applying a little solder directly to the iron will keep the iron in good working order.

Solder spraying

With the increasing interest in metal spraying guns, it is well to consider their possibilities and their use. The spraying of solder into dents which, because of difficulty in removing upholstery might otherwise be very costly to repair, is increasing in popularity with motorists and service men.

The gun here illustrated incorporates a number of improvements in this kind of equipment. The flow of current is automatically regulated in this metal sprayer to maintain the gun at the most efficient temperature. The heating element is large enough to enable continuous operation, yet the thermostat automatically prevents overheating ; and fluctuation in line voltage does not affect its operation.

The following steps show how repairs are made by metal spraying :

  1. Plug the spray gun into a proper electrical outlet and connect up the air line so as to obtain about 65 pounds per square inch pressure at the gun ; then proceed as follows :
  2. If the dent is deep, bump or pull metal as nearly back into place as practicable so as to reduce amount of necessary lead to a minimum.
  3. Clean whole surface to be filled thoroughly with disk sander, wire brush, etc. Be sure that entire surface is down to bright metal and is free from oil or foreign matter.
  4. Stir the Cold Tinning Compound thoroughly with a clean wooden paddle ; then apply a small amount to the cleaned surface with this paddle.
  5. Work compound over entire surface with a clean damp rag, supplying additional compound when necessary until bright lustrous surface appears under the compound.
  6. Wipe excess compound away with a clean rag. Wipe from centre of area toward outside so as not to bring dirt or oil on the surface from the uncleaned edges.
  7. Place nozzle orifice on a flat surface and pull the air trigger. This diverts the air up through the solder well, blowing it out so that the solder will flow freely.
  8. When gun is hot enough to spray, pull the air trigger back as far as possible, then feed a stick of solder down through the solder well (do not force it), and direct the spray straight (not at an angle) against the tinned surface, holding tip of gun about 2 inches from work.
  9. First spray a light coat of solder over the entire tinned surface ; then continue spraying until all of the low places have been built up slightly higher than the desired surface.
  10. When spraying is stopped, even temporarily, remove solder stick and continue air full force until solder well is cleared of all molten solder. Then blow out well.
  11. With a heavy sanding disk (No. 16 grit suggested) or a body rasp, dress the lead down gradually to the desired surface. (Do not sand in one spot so long that the lead becomes heated.)
  12. Smooth the surface with fine sandpaper and finish for painting.

Spraying over a glazed surface

Sprayed solder has a dull, grey appearance with a rough surface texture, and as long as this surface is maintained, additional solder can be sprayed on it. However, if this surface has been glazed by filing, sanding, or by heat, it is necessary to soften it with metallic mercury before sprayed solder will stick to it. This can be done as follows :

Place a few drops of metallic mercury (quick silver) on the glazed lead surface and rub it over the desired area with a clean rag. At first the lead will take on the appearance of a bright tin but this will gradually dull as the lead is softened. Increase the air pressure to 85 or 90 pounds, and proceed to spray in the usual manner.

Quite often in sanding, the higher points of the lead will be sanded down to the desired level and low spots will appear which have not been touched by the sander. In such a case, the solder can be sprayed into these low spots without further treatment. Solder which penetrates the unglazed low spots will stick, whereas that which falls on the unsoftened glazed surface will come up upon sanding.

Filling up holes

In repairing holes or torn places in panels, gas tanks, etc., insert a small ball of steel wool in the hole just below the surface and fill and dress the dent in the regular manner.

Spraying on aluminium

Aluminium surfaces are more difficult to tin than steel, so that more compound must be used and it will be necessary to rub the compound more vigorously. At first the surface will turn black, but upon continued rubbing and application of the compound, the black residue will be wiped away and a bright tin obtained. Solder can now be sprayed on in the regular manner.