How to retrieve missing vertices from STL file I was coloring my object A and found this piece in blue has large area of missing vertices


I still have the original STL file



My plan was to segment those missing vertices from original uncolored object (call it B) and paste back (or merge) to A. I import the STL file to my project, I can do Shift+D to duplicate, but they are on separate objects/canvas, there seems to be no "paste" and Ctrl+j won't work...

Please help me to recover those vertices. Thanks!
Verify that you have not restricted painting to certain faces.  Blender has elaborate tools for restricting access.  Face Selection





Verify you have not hidden vertices with the letter H.  See the Blender Menus to Hide and Unhide.  This is in Edit Mode.  
Inspect a render to see if vertices are missing


After the above steps consider joining.


Be in object mode.
Make sure you select both objects, then join.  
Join of two or more [selected] mesh objects in [object mode] works.


Selection in Object Mode.  Select Multiple Objects


Shift Click.  
Letter B to rectangle select.
Letter C to select by the centers of objects.


I you have not invested too much time in painting consider starting fresh with the correct STL file. (Delete the damaged mesh) Maybe your painting skills are faster than vertex joining skills.

Joining the vertices in recovery mode seems a challenge.  Blender has many vertex fill tools.