Car Body Solder - Instructions on How to Do It
Have you ever wondered what the professional body repairer did before the days of plastic filler? In fact he did use a 'filler' of sorts for smoothing out ripples and shallow dents, and that filler was a far stronger substance than today's more common substitute.
Old-style filler, known as body solder, consists of an alloy of lead and tin and has the effect of strengthening repairs, unlike plastic filler which simply sits on top of them.
Old-style filler, known as body solder, consists of an alloy of lead and tin and has the effect of strengthening repairs, unlike plastic filler which simply sits on top of them.
Most people have seen molten electrical solder and will know that it runs as freely as water. How then can body solder be persuaded to stay on vertical surfaces without simply running in an expensive stream onto the floor? The answer lies in the properties which tin and lead develop once they are alloyed together.
Tin has a melting point of 450°F (232°C) and lead melts at 620°F (377°C). Whenever tin and lead are mixed together, however, the melting point of the resulting alloy is considerably lower than that of lead.
Also, body solder, having a combination of 30% tin and 70% lead (and thus commonly known as 30/70 solder) has the remarkable property of going into a plastic almost putty-like, state at around 360°F (180°C) and staying like that right up to a temperature of around 500°F (260°C) at which temperature it turns to a liquid.
This means that body solder can be spread around like butter if it is kept within this temperature range, but of course, there is rather more to it than that...
Tin has a melting point of 450°F (232°C) and lead melts at 620°F (377°C). Whenever tin and lead are mixed together, however, the melting point of the resulting alloy is considerably lower than that of lead.
Also, body solder, having a combination of 30% tin and 70% lead (and thus commonly known as 30/70 solder) has the remarkable property of going into a plastic almost putty-like, state at around 360°F (180°C) and staying like that right up to a temperature of around 500°F (260°C) at which temperature it turns to a liquid.
This means that body solder can be spread around like butter if it is kept within this temperature range, but of course, there is rather more to it than that...
Where to Body Solder
There's no doubt that body solder is greatly superior to plastic filler in many respects though its advantages are often exaggerated. Plastic filler has earned some of its bad reputation for the paradoxical reason that it is so good!
It can be used over any surface and so it's frequently used to patch rusty holes which, of course, break right through again. Body solder can’t be used in this way.
When plastic filler is used over totally sound metal however, it has excellent qualities. So, unless you are an out and out traditionalist, you will probably want to use body solder only in those places where it will do most good.
The joint between two panels is one ideal place because the solder will run into the joint and both strengthen it and - more importantly—help to seal it against further corrosion. Another perfect place is where steel has become pitted on one side through corrosion, without having broken through.
Here, the corrosion is sandblasted clear then the pitted area built up with body solder. A third area which is perfect for the use of the material is where a corner of a bonnet, or some other small projection, is completely missing.
Again the area has to be completely cleared of any corrosion and this time a great deal of solder has to be used in one spot in order to build up sufficient material.
It has been emphasized several times already that the surface of the steel has to be thoroughly cleaned before soldering. This is because the solder will just not stick to steel that is not both chemically and physically clean.
One of the strengths of solder is that it combines chemically with the surface of the steel, while plastic filler just sticks on top. If any contamination is left, it forms a barrier which prevents the process taking place.
It can be used over any surface and so it's frequently used to patch rusty holes which, of course, break right through again. Body solder can’t be used in this way.
When plastic filler is used over totally sound metal however, it has excellent qualities. So, unless you are an out and out traditionalist, you will probably want to use body solder only in those places where it will do most good.
The joint between two panels is one ideal place because the solder will run into the joint and both strengthen it and - more importantly—help to seal it against further corrosion. Another perfect place is where steel has become pitted on one side through corrosion, without having broken through.
Here, the corrosion is sandblasted clear then the pitted area built up with body solder. A third area which is perfect for the use of the material is where a corner of a bonnet, or some other small projection, is completely missing.
Again the area has to be completely cleared of any corrosion and this time a great deal of solder has to be used in one spot in order to build up sufficient material.
It has been emphasized several times already that the surface of the steel has to be thoroughly cleaned before soldering. This is because the solder will just not stick to steel that is not both chemically and physically clean.
One of the strengths of solder is that it combines chemically with the surface of the steel, while plastic filler just sticks on top. If any contamination is left, it forms a barrier which prevents the process taking place.
How to Body Solder
You will need a few sticks of 30/70 solder, a pot of solder paint and a brush. Also get a stainless steel spatula used for spreading the solder.
Each of these materials can be purchased individually from paint stores, and a spatula could be made from a piece of smooth wood. Since tin became one of the world's semi-precious metals, body solder has become expensive.
Each of these materials can be purchased individually from paint stores, and a spatula could be made from a piece of smooth wood. Since tin became one of the world's semi-precious metals, body solder has become expensive.
This is an ideal place for the use of body solder. Here a classic car's wing has been repaired using three separate repair sections. One is a replacement wheel arch panel while the others complete the front and rear of the wing.
Each panel was joined to the others via a 'step’ in the adjoining panel so that there was a small overlap but the panels lay flush with one another. These left panels which lay beautifully flat and true but with joints that would be susceptible to corrosion in several years time, unless something was done about it.
That something, at least as far as the outsides of the panel were concerned, was to lead load it. Before that could be done, the joint was thoroughly cleared of any of its protective paint with a spot sandblaster run from a standard compressor.
This scoured out all of the paint right from inside the joint and also removed any traces of rust that might have developed. If no sandblaster is available, it is important to spend some time scraping and sanding every bit of corrosion out of the joint, otherwise the solder just won't take.
The paint also was cleared off, about an inch or so back from the joint, using a medium grit sanding disc, sanding until all traces of paint were removed. Paint can also be removed by heating it until it curls and then scraping off while it is still soft.
Either the welding torch can be used with a soft flame to remove the paint or a butane torch, the sort that is mounted on top of a disposable cartridge, could be used. Quite often, when metal has been dented or rippled it is extremely difficult to sand all of the paint or corrosion off the surface.
A useful tip is to take a sanding disc and cut it into a square or an octagon shape. Then the 'points' of the shape will reach down into the concave parts of the panel. Old-timers would 'tin' the surfaces of the steel before starting to load the solder.
This has the effect of putting a thin coat of solder all over the surface onto which the solder can be built up. Steel does not have the same ideal affinity for solder as copper, for example, so the tinning process must be carried out thoroughly.
The old-style way of doing it was to coat the surface of the steel in flux first of all. Flux is necessary to stop the surface of the steel going black as it oxidizes (reacts with the air); it melts at a lower temperature than solder, runs over the surface and keeps the air out long enough for the solder to flow over the steel and combine with it.
Each panel was joined to the others via a 'step’ in the adjoining panel so that there was a small overlap but the panels lay flush with one another. These left panels which lay beautifully flat and true but with joints that would be susceptible to corrosion in several years time, unless something was done about it.
That something, at least as far as the outsides of the panel were concerned, was to lead load it. Before that could be done, the joint was thoroughly cleared of any of its protective paint with a spot sandblaster run from a standard compressor.
This scoured out all of the paint right from inside the joint and also removed any traces of rust that might have developed. If no sandblaster is available, it is important to spend some time scraping and sanding every bit of corrosion out of the joint, otherwise the solder just won't take.
The paint also was cleared off, about an inch or so back from the joint, using a medium grit sanding disc, sanding until all traces of paint were removed. Paint can also be removed by heating it until it curls and then scraping off while it is still soft.
Either the welding torch can be used with a soft flame to remove the paint or a butane torch, the sort that is mounted on top of a disposable cartridge, could be used. Quite often, when metal has been dented or rippled it is extremely difficult to sand all of the paint or corrosion off the surface.
A useful tip is to take a sanding disc and cut it into a square or an octagon shape. Then the 'points' of the shape will reach down into the concave parts of the panel. Old-timers would 'tin' the surfaces of the steel before starting to load the solder.
This has the effect of putting a thin coat of solder all over the surface onto which the solder can be built up. Steel does not have the same ideal affinity for solder as copper, for example, so the tinning process must be carried out thoroughly.
The old-style way of doing it was to coat the surface of the steel in flux first of all. Flux is necessary to stop the surface of the steel going black as it oxidizes (reacts with the air); it melts at a lower temperature than solder, runs over the surface and keeps the air out long enough for the solder to flow over the steel and combine with it.
Here, in the picture, you can see an alternative to flux and stick solder being used. Solder paint is being brushed onto the surface of the joint. This is simply flux in which powdered solder is held in suspension.
Naturally it has to be stirred well before use because the solder grains tend to sink to the bottom, then it should be painted onto the surface fairly heavily. The next step is to play a flame over the solder paint until the solder 'flushes'. In other words, the solder has to be melted at which point it flows across the surface of the steel.
The point at which this takes place should be fairly obvious because the dull matt grey of the solder paint will be replaced by the silvery gleam of fresh solder. A common mistake when carrying out this job for the first time, especially for those who are used to welding, is to apply too much heat too rapidly.
The biggest risk comes when a welding torch is being used, simply because there is so much heat 'on tap'. If the metal is overheated, the flame will burn the flux away and oxidize (blacken) the surface of the steel before the solder has had a chance to melt.
If you intend using a welding torch, choose a medium nozzle, have a 'soft' flame (oxygen turned down a little) and play the heat lightly over the job from a respectful distance, not up dose such as when you are welding.
No matter how hard you try to dean the metal, there will almost certainly be some impurities left—so it's a good job that flux has a slight cleaning action to go with its ability to keep oxygen out.
As a result, the flux throws up a small quantity of black waste onto the surface. This must be removed thoroughly before attempting to add any more body solder. The flux itself is water soluble and so it can be removed along with the waste by scrubbing it with a wet rag.
The next stage is just a little bit tricky to master and involves one of those sets of actions where you have to do and think about two different things at once. After a while, of course, it becomes automatic—do you remember the first time you drove a car and how you seemed to have to do so much all at the same time?)
Naturally it has to be stirred well before use because the solder grains tend to sink to the bottom, then it should be painted onto the surface fairly heavily. The next step is to play a flame over the solder paint until the solder 'flushes'. In other words, the solder has to be melted at which point it flows across the surface of the steel.
The point at which this takes place should be fairly obvious because the dull matt grey of the solder paint will be replaced by the silvery gleam of fresh solder. A common mistake when carrying out this job for the first time, especially for those who are used to welding, is to apply too much heat too rapidly.
The biggest risk comes when a welding torch is being used, simply because there is so much heat 'on tap'. If the metal is overheated, the flame will burn the flux away and oxidize (blacken) the surface of the steel before the solder has had a chance to melt.
If you intend using a welding torch, choose a medium nozzle, have a 'soft' flame (oxygen turned down a little) and play the heat lightly over the job from a respectful distance, not up dose such as when you are welding.
No matter how hard you try to dean the metal, there will almost certainly be some impurities left—so it's a good job that flux has a slight cleaning action to go with its ability to keep oxygen out.
As a result, the flux throws up a small quantity of black waste onto the surface. This must be removed thoroughly before attempting to add any more body solder. The flux itself is water soluble and so it can be removed along with the waste by scrubbing it with a wet rag.
The next stage is just a little bit tricky to master and involves one of those sets of actions where you have to do and think about two different things at once. After a while, of course, it becomes automatic—do you remember the first time you drove a car and how you seemed to have to do so much all at the same time?)
The blowtorch has to be used to heat the panel over an area which covers no more than, say, the size of a playing card. At the same time, the solder has to be held on the edge of the flame so that it is being pre-heated, but not by enough to melt it.
When, after a minute or two, you judge that the panel is hot enough, try pressing the end of the solder stick onto the joint, if everything is ready, the solder will become droopy and waxy and at the same time stick to the tinned surface of the steel.
You should now try to deposit ‘dollops' of solder at dose, regular intervals along the joint or across the surface of the panel. Don't even begin to think about making a smooth surface; all you are doing at this stage is heating the solder to its plastic stage (between 360-500° F; 180-260°C), and depositing the material onto the panel.
The next stage is the one where you smooth the solder out. Throughout this, and indeed throughout the previous stage, it is best to have a piece of steel on the floor beneath the area being soldered.
You can waste an awful lot of the stuff, especially if you are inexperienced; as has been pointed out before, it's very expensive. When you have finished, collect together all the splash shaped scraps and store it.
When you have enough, make up a mold from a piece of right-angled steel and blank the ends off with lumps of ordinary household glazing putty. Put the solder scraps into a discarded can and grip the edge with a self-grip wrench.
Now heat the bottom of the can with the butane torch or with a very soft oxyacetylene flame (keep it moving so as not to burn through the can) then, when the solder is melted pour into your mold.
Hey presto, you've got a 'free' solder stick! And you'll be amazed at how much you can save!
The spreader (or paddle) used for spreading the solder can be stainless steel, although old-timers used hardwood paddles made of beech or boxwood which they kept smooth and burnished with oil.
The beginner tends to get the paddle into the flame, so perhaps stainless is best - it doesn't burn. If you look at the range of temperatures between which solder is soft, it looks pretty wide, but in practice, the range seems narrow when you're actually holding the torch.
Heat a blob of solder and hold the paddle dose by. Periodically, remove the flame and press down on the solder with the paddle. At first the solder will start to move 'but in a rather crumbly way; heat it for a little longer and it will spread like butter on a summer's day.
Heat it too far, however, and it will slip in a silvery stream onto the floor—a demoralizing sight! Again, don't worry too much about having a smooth finish at this stage. Try for a consistent, even thickness which is slightly proud of the surface you want to finish up with.
The solder can be filed down using a body file, which is a single-cut file and thus one which resists dogging. It is easy to take off too much of the solder, especially if the file is new, because of the softness of the solder relative to the hardness of the surrounding steel.
Professional body repairers often keep a semi-blunt file solely for filing body solder, just to prevent digging-in. An important safety point here is that body solder should never be removed with a power sander.
The lead would become air-borne dust which could then be inhaled with HIGHLY INJURIOUS CONSEQUENCES! Filing should be done from all angles, working from the outside edges of the soldered patch if it is a large one, and working inwards.
Long, smooth strokes should be used wherever there is room and just enough pressure should be used to prevent the file from skidding over the surface without touching. Final sanding of the solder should be carried out with 80 or 100 grit paper.
Where the solder is found to be a little low, you have three options open. You can follow a risky, perfectionist's path and attempt to build up more solder. You can console yourself with the thought that the joint is sound and strong and finish off with a skin of plastic body filler.
Or, if the depression is really shallow, you could follow the path of the old timers and use a skin of cellulose putty although really this is inferior to plastic filler with no real advantages.
Finally, a tip worth remembering in connection with the heat input involved. When applying body solder to a large, flattish panel such as a door skin for example, it is easy to cause heat buckling, especially if a wide area has to be covered.
Buckling can easily be prevented by having a pail of water and a rag to hand and by soldering a small area at a time then quenching it with the rag afterwards. This restricts the flow of heat through the panel and shrinks the localized area back down to its original size, provided that it has not been expanded too far.
When, after a minute or two, you judge that the panel is hot enough, try pressing the end of the solder stick onto the joint, if everything is ready, the solder will become droopy and waxy and at the same time stick to the tinned surface of the steel.
You should now try to deposit ‘dollops' of solder at dose, regular intervals along the joint or across the surface of the panel. Don't even begin to think about making a smooth surface; all you are doing at this stage is heating the solder to its plastic stage (between 360-500° F; 180-260°C), and depositing the material onto the panel.
The next stage is the one where you smooth the solder out. Throughout this, and indeed throughout the previous stage, it is best to have a piece of steel on the floor beneath the area being soldered.
You can waste an awful lot of the stuff, especially if you are inexperienced; as has been pointed out before, it's very expensive. When you have finished, collect together all the splash shaped scraps and store it.
When you have enough, make up a mold from a piece of right-angled steel and blank the ends off with lumps of ordinary household glazing putty. Put the solder scraps into a discarded can and grip the edge with a self-grip wrench.
Now heat the bottom of the can with the butane torch or with a very soft oxyacetylene flame (keep it moving so as not to burn through the can) then, when the solder is melted pour into your mold.
Hey presto, you've got a 'free' solder stick! And you'll be amazed at how much you can save!
The spreader (or paddle) used for spreading the solder can be stainless steel, although old-timers used hardwood paddles made of beech or boxwood which they kept smooth and burnished with oil.
The beginner tends to get the paddle into the flame, so perhaps stainless is best - it doesn't burn. If you look at the range of temperatures between which solder is soft, it looks pretty wide, but in practice, the range seems narrow when you're actually holding the torch.
Heat a blob of solder and hold the paddle dose by. Periodically, remove the flame and press down on the solder with the paddle. At first the solder will start to move 'but in a rather crumbly way; heat it for a little longer and it will spread like butter on a summer's day.
Heat it too far, however, and it will slip in a silvery stream onto the floor—a demoralizing sight! Again, don't worry too much about having a smooth finish at this stage. Try for a consistent, even thickness which is slightly proud of the surface you want to finish up with.
The solder can be filed down using a body file, which is a single-cut file and thus one which resists dogging. It is easy to take off too much of the solder, especially if the file is new, because of the softness of the solder relative to the hardness of the surrounding steel.
Professional body repairers often keep a semi-blunt file solely for filing body solder, just to prevent digging-in. An important safety point here is that body solder should never be removed with a power sander.
The lead would become air-borne dust which could then be inhaled with HIGHLY INJURIOUS CONSEQUENCES! Filing should be done from all angles, working from the outside edges of the soldered patch if it is a large one, and working inwards.
Long, smooth strokes should be used wherever there is room and just enough pressure should be used to prevent the file from skidding over the surface without touching. Final sanding of the solder should be carried out with 80 or 100 grit paper.
Where the solder is found to be a little low, you have three options open. You can follow a risky, perfectionist's path and attempt to build up more solder. You can console yourself with the thought that the joint is sound and strong and finish off with a skin of plastic body filler.
Or, if the depression is really shallow, you could follow the path of the old timers and use a skin of cellulose putty although really this is inferior to plastic filler with no real advantages.
Finally, a tip worth remembering in connection with the heat input involved. When applying body solder to a large, flattish panel such as a door skin for example, it is easy to cause heat buckling, especially if a wide area has to be covered.
Buckling can easily be prevented by having a pail of water and a rag to hand and by soldering a small area at a time then quenching it with the rag afterwards. This restricts the flow of heat through the panel and shrinks the localized area back down to its original size, provided that it has not been expanded too far.
Soldering Aluminum
Aluminum can be soldered using ordinary 30/70 solder but it has to be prepared in a rather different way. It is possible to buy a special bar of solder which has to be used for tinning the aluminum first.
The surface of the aluminum has to be thoroughly cleaned up first in the usual way and then the special tinning bar is melted onto the surface of the metal. Next, a slightly strange process has to be carried out.
Whilst the tinned surface is kept molten, a sharp tool such as a scriber has to be scratched vigorously all over the area which has been tinned, reaching through the molten solder and scratching through the surface of the aluminum beneath.
This has the effect of scratching away the outer layer of the aluminum while giving oxides no chance whatever to form and allowing the special tinning aluminum to combine with and key into the metal.
From then on, the process is exactly the same as in using body solder on steel and exactly the same materials are used.
The surface of the aluminum has to be thoroughly cleaned up first in the usual way and then the special tinning bar is melted onto the surface of the metal. Next, a slightly strange process has to be carried out.
Whilst the tinned surface is kept molten, a sharp tool such as a scriber has to be scratched vigorously all over the area which has been tinned, reaching through the molten solder and scratching through the surface of the aluminum beneath.
This has the effect of scratching away the outer layer of the aluminum while giving oxides no chance whatever to form and allowing the special tinning aluminum to combine with and key into the metal.
From then on, the process is exactly the same as in using body solder on steel and exactly the same materials are used.
