Solder and flux for brass radiator tank

I have taken the top tank off a 1953 Ferguson tractor radiator to clean and repair the core . All seems to be going well with the brass tank cleaning up well and the core having only a few small leaks .
What to use as solder and flux to re solder the tank back into position is the question , I have soldered before but mainly electrical work and on lead light windows with an electric iron .
I have a decent propane torch and usually use a 50/50 mix tin- lead solder with tallow as a flux . This works reasonably well in most applications but I think I may need a flux that is a little more aggressive and perhaps a slightly harder solder . Suggestions are very welcome .

I've used what ever solder I had on hand, 60/40, 50/50, ect... I typically use an acid flux like you would use for soldering copper pipe together in a house. Something that contains zinc chloride.

An old guy I knew could solder anything and he used "Cut Acid" I didn't know what that was and one day he was fixing a heater core for my '35 Ford. He took some Hydrochloric acid and cut up an old zinc jar ring and dropped the pieces into the acid. When the pieces had dissolved he brushed the liquid onto the area to be soldered and everything flowed perfectly and the repair was complete!...so Zinc is needed in the mix somewhere

I use 50/50 solder and tinning flux. you can get the flux at any plumbing supply or good hardware store. The key is to have it very clean. Use a small torch tip and keep the heat moving.
Don't hold the heat in one spot.
I hope this is helpful. Al

Thanks men , all good advice . I had forgotten about the acid /zinc trick . It is called ' killed spirit ' here , I haven't heard that term being used since the early seventies . Paste flux is probably the most accurate to place at least , I will most likely use a small brass brush on a Dremel tool to clean up the areas first.
What are your thoughts on further cleaning the surfaces with acid before applying flux ?

When I was an apprentice sheet metal worker w used a commercial flus called RUBY FLUX to solder copper domes on buildings. At home I have used muratic acid and killed it by dropping galvanized nails into the acid jar. Now I am not sure the galvanized nails have pure zinc for the coating, Modern galvanized nails look different than they did 49 years ago. P.S. Muratic acid is commercial grade sulphuric acid

Thanks Hillside , I think Muriatic acid is really Hydrochloric acid though . Called 'Spirits of Salts ' down under.

I used to rebuild and make new reproductions for early radiators. I used a 30/70 eutectic solder made especially for radiator work with an organic flux. It works a lot better than plumbing solder. Of course, even with the best stuff it has to be very clean to get a good joint.

Thanks again . Here is the tank stripped and ready to go back on . Radiator recoring is an almost nonexistent service here , limited to restorers of million dollars vintage cars mainly . The ' estimate' to recore the Ferguson radiator was $650 plus Aus . Worst thing was I am fairly sure it doesn't need a recore at all , after taking off the tank and cleaning each tube there appears to be only one small 1 inch section of one tube that has a few pinholes . The rest appears to be sound and soild apart from one area of physical damage .
Any tips on soldering the tube through the fins would be great .

Just block off the offending tubes at the ends. As long as they are few, and the others clean, it should be OK. Supplies should be easy to find in town for you, all the new cores i used came from Aus. and N.Z.

As others have said clean, clean, clean, which it looks like your are there.

Your torch is fine.

Next is flux, flux, flux. Preheat the components as you move along the seam, positioned as you want them to be after joining.
Apply your acid flux and spread it with a brush.....bristle brushes made for that purpose. Get them where you get your flux. Here
they are black bristles with a rolled tin handle. Use the brush to dab in the flux and spread it out as it melts; just enough heat, back
and forth in a waving motion to keep everything in a liquid state.

Come back with your solid core solder, 50-50 is fine, placing the solder on the metal and swiping heat back and forth across the
area as you move down the tank. When at the right temp, the solder will melt from the roll end and make a hump but continuing to
swipe the heat it will puddle and flatten out, SMOOTH AND SHINY; no rough, sharp point, grainy appearances......that's a "cold
solder joint". Melt some solder and immediately brush it back and forth getting good adhesion to both metals. Once you have a
nice shiny surface, apply more solder, still swiping the flame back and forth to build up a detectable thickness.

That's how I do it.

Don't use rosin core electrical solder. You need the cleaning that only acids provide.

Good Luck,
Mark

I use acid core solder on radiators and gas tanks.