Was curious to see what you guys use?
The plain old paste stuff you buy at places like Home Depot.
Plumbers soldering flux is what most would call it.
Works fine, no after corrosion.
Same rules apply...clean, clean surfaces & don't overheat the flux and 'burn' it.
I've always used it with lead/tin and the silver/tin soft solders alike without problems both for small attachments and full re-rib jobs.
I don't like using the zinc chloride flux (so called acid flux) for it's aggressive after rust tendency.
JMO of course.
Zinc chloride flux-- same hazards as welding/oxy-fuel cutting galvanized (zinc coated) steels--zinc poisoning is serious. RWTF
Unfortunately there is not a truly universal flux for soldering. Here are just a few I use.
Rosin based flux and pure Rosin works well for many situations, and a big plus non corrosive used a lot in the Electronics industry. also the preferred flux for rib soldering on new side by side barrels or relaying them.
Liquid type soldering fluid is good for tinning some steels and Iron but you must wash the residue away because it is corrosive, usually containing Zinc Chloride plus an acid
Zinc Chloride in Petroleum Jelly is also a good all rounder for tinning, downside the residue is also corrosive, this is one of the oldest commercially available fluxes for the hobbyist.
For stainless Steel I use Phosphoric Acid based fluxes, though not always successful on some steels.
Some of the self cleaning fluxes used in the plumbing industry are good for Copper & Brass, also work well when using Lead free solders, though not convinced about them being non corrosive.
Thought I should add this one, Borax is used for high temperature soft solders though these types of solders are not common . Borax is also good for very high temperature Brazing and Silver soldering
For lead/tin soldering, I've never had any success tinning bare steel with rosin flux. I tin the components to be soldered with acid based flux, scrub them thoroughly with a scouring pad and soap, then solder the tinned parts with rosin based flux. Same for using Brownells HiForce 44.
I struggled with rosin flux for a long time until it finally clicked for me. The temperature envelope is relatively small and very critical with rosin flux. Too cold it will not bond, too hot and the flux burns and it will not bond. I use Kester liquid rosin flux and a 150 watt iron for most tinning work. On larger pieces I heat them with an acetylene b tank turbo torch.
For tinning I use acid based flux. If I am then rejoining ribs, the acid flux must be thoroughly cleaned off. Then I use straight pine tree rosin that I crush into a coarse powder.
I used to work with a VERY skillfull Metal fabricator who was Japanesse, I asked him to teach me his soldering skills and what type of fluxes he used . He said " Borax"