I've taken apart some of my own test welds, and while most of them look great (solder the length of the joint) every once in a while I do one that I know is bad before it even cools, because the hot solder just keeps rolling off the pipe instead of being sucked into the weld. When I take it apart to confirm, I see that there is no solder in the weld, just a small amount of black residue which I'm guessing is burnt flux.
Is that caused by something specific? Pipe too hot or too cold before welding? Is it something that can be fixed if the weld is in place, or will I have to always separate the joint and re-clean it, re-flux, and re-weld? I'm thinking if the pipe is too hot, I should be able to let it cool and then just try to scrape in some flux (it would get sucked in as the pipes re-heat and it melts) and re-solder.
I'm thinking especially if this happens on a joint where I _can't_ tell it is bad until it is too late (hard to reach valve installations, etc) that it would be nice to be able to just fix it in place- I'm working on a really old house and trying to put valves in-line in a couple of places I can barely reach, and I'm not sure I'll be able to see well enough to be certain that it is ok, and getting a valve back out to clean it will mean taking apart 2 joints, doubling my chances of any errors when putting it back (versus just fixing one weld).
Any advice welcome (other than "hire a pro", I wish I could afford ti but I can't). Thanks! Keith