tinning steel

My first port of call would be plumbers flux. Though I guess they're not all equal.

I recall an instance when I attached a copper pipe to a club hammer I was using to support the pipe whilst soldering a joint.

Personally, I'd be tempted to use a gas torch and an active flux.

Phosphoric acid is good.

John

I've got some 80% strength what dilution? Why would it be better than hydrochloric acid?

I am thinking of unearthing a set of bodywork books from the 1930s that my wife's granddad gave me shortly before he died in the 60s. He was an old school pane beater and before resin fillers they would beat out the dents and use lead to get the final finish as a skim.

Phosphoric will convert the oxide (rust) to an adherent layer of iron phosphate. Good if you are going to paint, less helpful if you wanted to tin. Hydrochloric if you want to pre-treat (active fluxes are doing the same thing).

I have used concentrated phosphoric acid as a flux when soldering stainless steel. It can spit a bit, so wear eye protection. It makes really nice joints with tin/lead solder. I haven't tried lead-free solder. Hydrochloric acid or chlorine containing salts are more likely to leave corrosive residues as chloride ions catalyse rusting.

Isn't that what 'Kurust' was intended for ?. That has been reformulated as a water-based compound and is nowhere near as good as the original product.