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Ultimate Guide To Lead Loading

body solder lead loading

(Words and Photos: Nigel Boothman, Practical Classics Magazine, January 2008 issue)

Recently, Practical Classics has touched on various welding techniques, metal bashing and panel making, but often even the neatest repair needs to be filled before it’s worthy of the spray gun.

The same goes for tarting-up minor paint blisters and rust spots – filler is a fact of life. But who says it has to be plastic?

Used on bare or pitted metal, or even a primed surface, it usually just delays the inevitable and the repair can be bubbling up again within months.

That’s because most fillers and primers aren’t waterproof, and if you’re working in a damp atmosphere (also known as Britain) the chances are some moisture will find its way in.

Body solder, on the other hand, is totally waterproof. It’s what everyone used until plastic filler began appearing in the Fifties, and many manufacturers used it for years after that. It’s heavy but it’s capable of covering a multitude of sins with permanence and flexibility – it responds to dents and bangs better than plastic filler too. But it’s tricky and time consuming, isn’t it?


What’s in the kit? body solder lead loading kit

I thought it was time to find out whether you can just buy the bits and teach yourself, so I got a Standard Solder Kit from and got stuck in. The results were surprising.

Inside the box I found an adjustable file, two wooden paddles, a pack of brushes, two sticks of solder, some flux paint, a pot of tallow and a book that includes plenty of theory as well as practical instruction.

However, there’s no direct relation between the book and the other bits in the box.

For instance, does this pot labelled in the kit as ‘solder paste’ also do the job of a flux? It does, but I was confused by the book’s dealings with different products not relevant to the kit.

Before I charged ahead with the repair, I did a quick web search and found two very useful sites to do with lead loading – Tommy Sandham’s instructive page at, which took me in turn to a video of the lead loading process hosted by Sovereign Coachworks. Both gave me more confidence about the order in which it’s all meant to happen.


A man who knows

Leeds College of Technology runs occasional one-day courses on lead loading, and the tutors there are fonts of knowledge. I should have spoken to the college’s very own restoration expert Mike Coman before I began, as I could have learned some important tips:

‘Tinning thoroughly is crucial,’ he says. ‘Do an area a good two inches larger than the repair so you can spread the solder smoothly and be confident it’ll adhere. We’ve tried the newer lead-free tinning pastes and they work fine. Once you come to the lead application itself, look for a slight change in appearance that tells you when the solder’s softening.’

Some people have used electric heat guns to melt the solder, apparently with success, but Mike favours the old-fashioned approach. ‘Get a simple blowlamp from a DIY store – it’ll be more controllable than the hotter flame from an oxyacetylene kit,’ he says.

‘When you start filing, move the file diagonally across the work area with the front edge of the file angled up slightly. A flexible file can be adjusted to match a curve in the bodywork.’

It’s ambitious to get a paintable surface just from filing and sanding the solder, not least because you shouldn’t use anything finer than 80-grit paper if you want to avoid lead poisoning – and even then be sure to wear a mask.

‘Modern plastic filler is excellent for minor imperfections,’ says Mike. ‘But before you apply it, be sure to clean the whole area with a phosphoric acid-based rust converter. Follow the instructions properly – any acid left on the panel will affect the paint.’ Uh-oh. Missed that bit.

See online for more on Leeds College’s courses.


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This is the front of my 1982 Alfa Romeo GTV, which has been ‘restored’ at least once already. But it’s Italian and it’s 25 years old, so there’s plenty of choice for rust repairs.

I removed the bonnet and the plastic headlamp and grille assembly – there will be heat involved in this process and I don’t want to risk damaging anything.

A poke with the screwdriver has revealed glassfibre filler and missing metal, which will have to be scrubbed back and repaired.

Here it is after a touch with the angle grinder’s wire brush wheel. These ragged edges will be cut back to thicker metal and to make a straighter line to weld to.

Notice the hole in the wing seam area on the left – also in need of welding.

We’ve dealt with forming panels and MIG welding in PC recently, so we’ll move on to the end result.

It’s just some 18-gauge steel cut and bent to shape, slipped inside the panel and lap welded. The ugly welds are deliberate to see how the lead loading copes. Honest!


body solder lead loading tinning1

body solder lead loading tinning2 body solder lead loading tinning3
The next step is to heat the solder paste mix. The flux singes to a brown colour after a few seconds under the flame and the solder in the paste visibly melts into tiny silvery spheres. Have a bucket of water next to you. You’ll need a damp cloth to wipe away the flux paste, giving it a nice, shiny surface. The water is also handy if you get careless with the blowlamp.


body solder lead loading heat1 body solder lead loading heat1 body solder lead loading heat1 body solder lead loading heat1

Reach for a stick of solder, which in this kit is the standard 30 per cent tin, 70 per cent lead ‘Plumber’s grade D’ used for car body repairs, and aim the flame at the tip of the stick.

Let a spot on the work area get hot, too. Twist off the soft end of the stick and repeat until you have lumps of solder all over.

You’re ready to begin the supposedly tricky part.

Move around the area, heating up the lumps you’ve applied until they reach a consistency similar to smooth peanut butter, and spread the solder around with the wooden paddle, first smeared into the tallow (which you should warm first) to keep it from sticking.

Quick-cool the hot solder with a wet cloth when you’re happy with it. If you’ve got the area too hot and the metal looks blue, you’ll have to remove the lot and start again for best results.

I underestimated how much paint to remove around the repair, so the blowlamp blistered it, which will have to be sorted out later.

The ‘mush’ pot is your friend. Place some clean cardboard under the work area and catch the solder that slides off – it’ll soon be cool enough to put into an old metal tin.

Re-heat and re-use it. It’s sometimes easier to pour it out of the tin than to scoop it out with a paddle.



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When you’ve smeared and shaped your area, reach for the body file with curved teeth that act more like blades.

It chatters across the surface to start with, but ease back on the pressure and it shaves a beautifully smooth finish into the solder. Save the filings for the ‘mush’ pot.

The other area at the front wing seam was filled purely from the filings saved in my trusty mush pot.

Hand sanding with a block and coarse paper leaves the front virtually finished – no power tools, remember, because the dust is highly toxic. Wearing a mask is essential.

Using plastic filler after all that effort might look like hypocrisy – but this is just a ghostly-thin layer of ‘stopper’, a very fine two-pack filler that you can smooth to an almost porcelain finish.

Don’t wet sand it – it’ll soak up water and then shrink later, ruining the job.

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A top tip is to blow a thin guide coat of primer over the area you’re working on to check for imperfections that will require further filling and sanding. Once you’re satisfied that you have a blemish-free treated area, two coats of primer and flatting with 1200-grit paper will leave it ready for the top coats – and a beautiful, long-lasting finish you’ll be proud of.


Here’s the surprise: lead loading is easy and it’s fun! OK, so it won’t be perfect first time out, but that’s more to do with finishing than with the process itself. As long as you start on a smallish repair on a horizontal surface, you’ll master it in no time.

It’s inspired me to get on with all the other body repair jobs that need to be carried out.


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