For ALL of my brass scratchbuilding, using either a torch, an iron or my resistance soldering station, I use Supersafe, Superior No. 30 Soldering Flux (gel) available from H & N Electronics here:
http://www.ccis.com/home/hn/It is the best thing that ever happened to my model soldering and it makes a HUGE difference in both the ease of soldering, and in clean-up...and also in the permanency of the joint...meaning ZERO corrosion after decades of being under paint.
Back in the 80's I started building my turnouts by hand, and doing a lot of engine mods on brass imports. I tried the traditional methods, meaning what I could buy at Ernst's (remember them?) and although there were several acid fluxes that worked GREAT in getting the parts to stick together and the solder to flow under minimal heat, I learned the hard way that even after washing my constructions in hot water, neutralizing with baking soda...that after a year or so, the joints started to corrode, and bubble up through the paint.
The problem is, especially with complex, layered assemblies that rinsing doesn't get it all. Yeah, Plumber's Honey (acidic flux) works great on copper pipes for soldering your toilet water supply line together, since water is continually washing it, but for model work, it turns into a nightmare, with lots of hard, precise work becoming ugly and useless over a bit of time.
Now, the vast majority of my soldering is for making turnouts, with the occasional detail part soldered together or stuck on a brass carbody or boiler. Supersafe Superior No. 30 self-neutralizing flux makes these operations exponentially easier than they were before. On some parts, I have not been able to rinse them, such as turnout repairs on the layout...and even after years, there is no sign of any corrosion.
Everybody I know who tries this flux says the same thing I do, and it becomes a main-stay of their modeling solder work.
As to types of solder...there's a huge difference between plain old lead solder and alloyed solders as far as strength is concerned. Also, solders melt at different temps, so if you solder with high temp silver-bearing solders first, you can add more details by using a lower temp solder for attaching them.
An excellent, well accepted and widely used low-temp solder is Tix solder, which is not silver bearing, but an iridium/tin/lead...and a few other metals...alloy. It melts at 275 degrees and is advertised as "the strongest soft solder in the world" with a strength of "4300 psi" whatever that means.
I also use H & N Electronic's 96/4 Tin/Silver solder, which melts around 460 degrees, and is advertised as being "five times" stronger than tin/lead solders...using both Tix and H & N's allows layering because of their much different melting temps.
Although Tix makes a good flux too, and is in use widely by the jewelry trade, I still use Supersafe Superior No. 30 for my model work.
On many of my brass models, I don't drill holes and insert grab irons into them...then solder. I use my American Beauty resistance soldering station to both hold and heat pre-bent and tinned grabs (with flattened ends) and solder them directly to the brass car sides or boilers. I have never had one break off yet using either the Tix or H & N siver bearing solders, and to my eye, they also look much more prototypical.
So, yup...if you're concerned about strength, I'd opt for the H & N or Tix solders, and flux 'em with Supersafe Superior No. 30. Using both gives a lot more versatility in assembling your brass pieces too.
Lastly, complex soldering of models composed of varying thicknesses of metals is a LOT easier with a high-quality resistance soldering station, with the "tweezer" and "probe" handpieces minimally. IMO American Beauty makes the best, and sometimes they're on sale at Micro Mark. The two features that make them light-years more advanced than either a torch or iron is that (1) you can hold the pieces together with the same instrument that supplies the heat...and you can (2) instantly back off the heat as soon as you see the solder flow by lifting off the foot pedal...and continuing to hold your assembly together for a few seconds until the solder solidifies with either the tweezer or graphite probe.
Good luck!
Bob Gilmore
