Being the king of do-overs, I assembled my shower valve and copper pipe and tested it yesterday and sure enough had a small leak in one of the solder joints at valve. I heated it up at the joint and pulled it apart. I have extra copper to refit that side of the valve so I'm not worried about that part too much (more practice for me
On the valve port itself, it looked pretty clean once I pulled the short piece of copper out - left couple of very tiny bits stuck on the inside surface near the edge of the valve port - I took some emory cloth and in a couple minutes I cleaned the valve out to the point where it looks ok to me (shiny, smooth inside) - didn't take much, and the new section of copper pipe fits in snug but not too tight.
Is there anything else I should do to prep the valve surface inside or can I just paste and solder as if nothing happened? In the few soldering jobs I've done before, I've only had one other leak and I just replaced the parts and did it again - easy and cheap. This time I have the valve to deal with. Hope I didn't fubar it.
Related question - is there a "due date" on solder or paste? the stuff I have is pretty old - probably 5 years. does it matter? Can I blame my crappy solder job on bad parts? guessing probably not....