I’ve sanded down to where the wall feels even but now the patch’s showing what should I do next?
Walls – Need some help with wall patch
patchwalls
Related Solutions
My first guess is that the threads where the valve is screwed onto the pipe have started to leak. Even with the water turned off, there will almost certainly be water in the line that would leak. With the water turned off, you can try unscrewing the valve from the wall (as long as the plumber didn't solder the threads, it should come off). If the valve is relatively new and the threads are in good condition, then you can clean the threads, apply plenty of pipe dope to the threads, and screw the valve back on.
Note that if you don't see any mineral buildup around the pipe or threads, then the problem may be inside the wall and require removing drywall to locate the problem. If that's the case, I wouldn't delay in opening things up, since a small sign of water outside could be concealing lots of water inside.
You can patch it with hydraulic cement similar to this type
You need to carefully remove all loose material and clean the area. The cement is fast setting and expands slightly as it sets, making a tight seal. Only mix as much as you can use in a few minutes.
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Best Answer
Part of the problem may be sanding technique. It appears that you have used a soft pad or bare sheet. You need to use a rigid block to prevent sinking into soft spots.
Apply another coat of patch material, and taper it out a bit further. Proper drywall work doesn't involve much sanding at all, so make sure to use a wide knife or trowel at a low angle to create a flat, smooth repair. Sanding should take mere seconds.