[mark_in_manc]

Thanks all. I obtained some SoS sold as drain cleaner and yes, it fumes like hell. It also came in the post, which surprised me as it is 25% HCl. So I'll have to try a little with some scrap wheel balance weights in (which when they are not steel, are now zinc thanks to control of lead regs) and compare the two.

I guess failing to neutralise the flux leads to corrosion - maybe a little alkali (I have some caustic soda) might be appropriate, though I can imagine an unstable feedback situation where my workpiece 'hunts' wildly around PH7 as I hit it with one after the other!