How to soft solder?

Not a how to, but a request to tell me how to.

I’ve been trying to soft solder mild steel and having zero luck! I’ve got good clean metal and can get the solder to adhere to one piece but not both. I wondered if it maybe my flux or solder but I can’t for the life of me find a recommendation of what others have used. Plenty saying to use silver solder but I don’t want to heat the steel to 600C, I’d be much happier at 400C as it’s been strong enough for most jobs over the last hundred years.

Any recommendations?

I'm by no means an expert but I have done some here and there over the course of my career.
My first guess is you don't have both pieces of metal hot enough. By the sound of it, one piece is hot enough and that's where the solder is sticking.

If one piece is considerably thicker than the other, focus your heat source on it first. Once it gets where you need it, start pushing that heat over to the thinner piece.

Long story short, the solder is going where the heat is and staying there.

I can only explain the full process of how I soft solder which works for me. I can't just explain part of the process, sorry.

For soft solder I would say your problem is not having the metal clean enough to start with and, if I was a gambling man, too much heat. Soft solder will only work correctly when the correct heat range is used, get it too hot and the joint starts turning black. Once the joint has turned black your efforts are wasted and no amount of jiggery-pokery will salvage the joint.

To start the process throughly clean both pieces of metal that you wish to soft solder so that they are shiny clean and you could use abrasive paper for this. Coat each surface that you wish to solder together and which you have just cleaned with an "active flux" suitable for soft soldering (this type of flux also cleans the metal as it gets hot). You then need to "tin" both surfaces with solder and you basically use just enough heat on the surface of the metal to melt the solder. If you do not first tin the surfaces with mild steel then, in my experience, you will never get them to soft solder together. Do NOT over-heat the metal, do NOT burn the solder and do NOT melt the solder onto the metal. Once both surfaces are covered in solder they both need to be "wiped off" while the solder is still hot leaving a thin even film of solder over each surface. I normally just use a cotton or non nylon piece of rag for this purpose. Hands should be protected with heat resistant gloves while the hot soldered surfaces are wiped down and care taken while this is carried out. All necessary personal protective equipment should be worn.

Once both surfaces have been properly tinned and "wiped off" apply further soldering flux to each of the surfaces that you have just tinned and if it is a wide joint you may wish to lay some small additional pieces of flattened solder, off the reel, onto the join. Place the 2 pieces of metal together which may also need some form of clamping. Next apply heat but do NOT over-heat and do NOT get the flame too close to the metal. Heat the join evenly, slowly and gently from a distance and once the flux starts to bubble and smoke you know that soon the metal will be up to temperature. Test the temperature of the metal by applying the solder but do NOT heat the solder onto the metal. Keep heating slowly and gently, over the area to be soldered, until the metal melts the solder and then back-off with the flame. Apply enough heat and solder to the joint as required but do NOT over-heat. If you over-do it with the heat and the joint turns black, no amount of additional heat, flux or solder will salvage the joint and it will be back to square one.

It is good practice to wipe down the soldered joint with a damp cloth when an "active flux" has been used but not everyone does.

Do please wear your personal protective equipment such things as gloves and googles.

Thanks gents, I’ve pretty much done as described except I think Flying10 has identified my issue, to much heat! My steel turns black so I shall try less heat and see if that works. I shall report back.

Do you guys just use regular solder?

I don't "overly worry" about what soft solder I use as fluxed and non fluxed will work. The variations of soft solder available will work at slightly varying temperatures. If you follow the correct soldering procedure most soft solders should work.

What is important, especially with steel and copper but also with other metals is to "properly clean" the metal first and this means that you do actually have to go over the surface with something like emery paper/cloth and or wire wool first. Just because the metal may look clean when you acquire it does not mean that you do not have to "properly clean" it first. Failure to "properly clean" the surface first will result in a poor join or no join.

To solder steel together the surfaces must be first "tinned", as already described. Failure to "tin" the surfaces of the steel first will result in complete failure of the attempt to solder the steel together.

Do not use too much heat and heat slowly and evenly using only just enough heat required to melt the solder. Using too much heat will completely destroy the attempt to solder. You are not attempting to replicate a blast furnace in your shed/workshop, your are soft soldering which melts at a relatively low temperature.

This was the best video that I could find to describe the "tinning process". Although the "tinning" in this video is for the purpose of lead loading a car body the process is the same as required for other purposes.

Notice the importance given to the cleaning of the metal and notice the "wiping off" of the tinned surface while still hot.

The "tinning compound" mentioned in the video is basically a mixture of flux and powdered solder.

How to Apply Lead Body Filler , Metal Prep and Tinning Part 1 - YouTube

This is my first post on this particular gun website, so be gentle.

There is a liquid compound that is sold as "Johnson's Tinning Compound". Been around forever and used mostly in auto body work. It's chief claim to fame is that it is absolutely guaranteed to get solder to adhere to steel. I've used it to secure Mauser rear sight bases and it works just fine.

I might be off on that name as it's been a few years, but any decent auto supply store should be able to locate it. I'd suggest trying it on a few scrap pieces of steel first to get the "hang" of it.

"Tinning compounds" are normally a flux of some description with a certain amount of powdered solder already added to the flux.

heat the work, not the solder.... patience is key. clean stainless wire brush to clean the surface.. good quality flux

A very old fashioned flux for soft soldering is zinc chloride. This is made by dropping bits of zinc into Hydrochloric acid until it stops reacting.. hence the old name of "killed spirits" (..from "Spirits of Salt" the old name for Hydrochloric Acid)..

It is neither gentle or pleasant, but it will usually work were others don't.. it does however corrode like hell afterwards, so the joint will need a good clean with hot water and elbow grease!

I learned to soft solder from my grandfather (born in 1894). He liked using soldering coppers (looks like short fireplace pokers with a fat head). We would heat them on the kitchen stove and you could solder 36" of thin metal until it was too cool to keep working. In the garage, he had a brass plumbers "furnace" basically a white gas pump up burner to heat them. He also had an electric iron about the size of a road flare. That would work quite well but once he was doing a rain gutter and the 100' 12 gauge electric cord ran out of amps to get it hot enough. My teen years as a "Pool Boy" we used Acetylene hand torches (American torch) for up to 3" copper pipes. That made a big difference. Propane would not work past 1.5" copper. We never used MAPP so I have no experience. I never soldered anything but sheet steel. Galvanized was tough, but a real acidic flux was the key. The most fun was the hard coal forge about the size of a small suitcase. Get that going and he would make chisels, hardened washers and yard tools. Ah, to be a kid again...

Dave

The steel must be bright and grease/oil free, use a good acid flux, use a decent sized copper soldering bolt, warm the steel with the gas, apply the flux, heat the copper soldering bolt (also fluxed) until the colour begins to change to a rippling light blue, tin the bolt with the solder and apply to the warm fluxed steel ensuring that the part is well tinned. Repeat for the second part. Clamp both parts in correct alignment and heat until the joint runs. Trim and refinish when cool. Use only tin/silver solder for parts that will be hot blued. Easy-peasy!

Fascinating thread with very good info. My thanks to all!

i use water spray to cool off the a rag the spray i dolder twicein the roll