I'm redoing my shower, all 1/2" copper pipes. I've done two couplers,
1 elbow, and 3 threaded connectors... and they all went fine. Just
one other elbow had a small leak. I tried at least 10 times last
night and continue to have leaks. Maybe I just need to try again
tonight with a fresh mind, but...

One problem I've got now is that the two ends going in to the elbow
have solder on them. I've sanded them until smooth (they are still
solder color, but smooth, is that okay?) and used a new elbow, and did
that a couple times last night, but still no go. Is it okay to sand
off the old solder and keep trying this way, or do I really need to
start with all new pipe? It is easy to keep using new elbows, I've
got plenty to spare... but I can't really cut back the pipe, not
without adding yet another fitting (another coupler) which just seems
like even more work, and eventually I'll have it cut back to the slab
and not be able to continue.

The elbow fits well, nice and tight, just like all the others I did,
so I have no idea why this one connection is giving me so much
trouble. I did only do one other elbow though, so maybe I just got
lucky on that elbow. Is there anything special you need to do when
doing elbows? I've tried doing one end at a time, and also tried
heating in the middle of the elbow and running the solder around both
ends, one right after the other (while still hot, so they both cool/
harden at the same time).

I've read a lot about soldering... but, how long do you need to wait
before testing? The pipe seems to cool pretty fast, so I've been
testing within 2 to 5 minutes, is that not long enough? I kind of
wish I could find an elbow with about 6" of pipe on each end, so I
wouldn't have to worry about messing up one end while working on the
other.