I am currently renovating my bathroom and recently removed the copper supply piping back to and including the hot and cold shutoff valves to repipe for a new shower valve. Everything was repiped with copper, cleaned, prepped and soldered including 2 new ball valves, a new shower valve and 2 new drop ears for shower and tub. I turned the water on and everything seemed to be good and none of my joints were leaking. I returned to double check the pipes the following day (about 10 hours later with pressure on all joints overnight) and there were no leaks, but I noticed on the "house side" of both shutoff valves there is the slightest amount of moisture on the surface of the solder joint between the shutoff valve and copper pipe. No dripping, not enough moisture to run down the pipe, just barely enough that when I run my hand across the joint I can feel and see the slight presence of moisture. It almost feels like condensation but I don't even want to call it that because the amount is so minimal. Is this a problem? Possibly usual for a new joints like this? This occurrence is happening on both valves and visually the joints look good so I don't believe it is a bad solder joint. Any help would be appreciated. I want to make sure everything is good before I install cement board and tile.
If there was water in the supply line when you went to sweat the joints the water turns to steam and can cause very tiny leaks like you have. I was taught to get as much of the water out that I could then pack some white bread (no crust) tight in the pipe. The bread makes a plug that once soldered a flow of water will dissolve (just need to remove the aerator) I do this on pipes that have been damaged and need a new coupling on both sides works great and no leaks, I believe this is the problem because the hot should never have condensation and if the water is not running the cold should not be much different than the room temp.
If it is on a valve, it could also be a packing nut leaking slightly. Often when installing valves on water lines (ball valve style) the packing nut doesn't come 100% tight and can seep water. These can typically be tightened with a crescent wrench.