I know ya'll can't see it, but I am getting a very, very, very
tight fit in the joint and I also round the edges of the joint
where the solder will enter.
Don't round the edges. The solder doesn't need that to flow in the
joint, and you're only making the joint wider at it's outside
surface, leading to a wider and more visible solder line, plus
increased potential for pits in the solder.
Then I flux the joint, making sure that flux wets the surfaces to
be soldered. I also use a small tack weld to hold the piece in
place. Then I coat the ring with a boric acid/alcohol mixture.
Wrong order. Boric acid first. Burn it off. THEN flux the joint. As
you're doing it, you've washed off the flux from the joint. That may
be your biggest problem. be sure too, that the metal is clean.
Nothing like beeswax from sawing, or other dirt, please, in the
joint. The flux too should be clean, not full of crud from long use
(applies more to jars of paste flux, which can accumulate assorted
debris over time on the bench) And use a decently active flux.
Batterns should be OK, but if you find trouble getting solder to
flow, try one of the paste fluxes like Dandix or Handy flux (try to
avoid active fluoride ones, though, unless you've the ventillation to
handle them). The paste fluxes tend to be more active, and help the
solder flow more aggressively than the self pickling ones like
Then I cut a strip of solder about 1mm wide and as long as the
shank is wide and set it on the joint on the inside of the ring.
Unless you've rolled the solder thinner than it normally is sold,
that sounds like more solder than you need. More is not better. Just
leads to more pitting, more clean up, and a more visible solder line.
When heating I play the flame across the joint (not parallel) on
the outside of the ring.
Remember that solder flows towards the heat. If you're heating from
the same side on which you placed the solder, it will just ball up,
or flow onto the top surface of the metal. To pull the solder into
the joint, heat from the other side of the joint as much as from the
top. Side to side or along the joint doesn't matter, so long as both
sides of the joint are equally heated. Again, remember that the
solder flows towards the heat. You can pull it where you like,
including into a joint, by choosing where to heat. Couple this with
proper fluxing, and you've likely found your problem.
The only thing that ever changes is my torch. Sometimes I use a
hydroflux torch and others I use a natural gas oxygen setup with a
Shouldn't make too much difference, though the flames aren't really
the same (the hydroflux type flames are smaller and more tightly
directed, so even heating of both sides of the joint needs to be more
deliberate. Use, with the natural gas, a slightly reducing flame, not
a strictly neutral one. Test this with a piece of copper, which will
remain clean and bright looking in a reducing flame as it heats up. A
neutral or oxidizing flame allows oxidation, sometimes in spite of
the boric acid or flux, and that will inhibit solder flow. With the
hydroflux unit, be sure to use the proper material in the vapor
fluxing unit, so it's flame too, ends up slightly reducing.
My end result is usually the solder just flows around the joint and
won't flow through the middle.
Usually this is a combination of not pulling the solder into the
seam with proper heat control, and oxidation that's preventing the
solder from flowing properly, which can be either an improper flame
adjustment, or improper fluxing, etc. You may wish to also try using
a solder poker to physically nudge the solder to bridge the gap in
the seam. Usually, when it's trying to misbehave and flow to one
side, if you can get it to start flowing into the seam by literally
pushing it in there with a poker, then once it starts, capillary
action will pull the rest of it in too.
Hope that helps.