How do I quickly mirror points?

Question

I've got an FBX file that was mangled pretty badly by an auto-export with downsampling. I'd like to clean it up by forcing vertices to be mirrored (it's supposed to be symmetric), but what I find myself doing is clicking one point, clicking the transform x-coordinate, hitting copy, clicking another point, clicking the transform x-coordinate, hitting paste, clicking the first point, etc. etc. for y- and z-axis values.

Some of the geometry is totally hosed and I'm finding I need to redraw it, but there again I'm clicking a point, ctrl+right click to put a new one down, then the back-and-forth transform dance to get it where it needs to go.

I found posts online about mirroring objects, but I can't seem to get anything reasonable to work for points.

What I'm actually trying to do is mirror faces; if there's a way I could just mirror faces too, that'd be even better. I also found references online about deleting half the mesh and adding a mirror modifier to create it, but the problem there is that the file was poorly downsampled and it's not mangled symmetrically (hence why it needs so much cleanup). I need some faces from one side, some from the other, etc.

Answer 1

Here's what I've been doing that works for me. Sorry I don't have pictures to go along with the post at the moment:

1. In Object mode, select the part I want to fix, Tab to Edit mode.
2. Hit 3 to go to faces mode, click faces that are blocking my view and hit H to hide them. Repeat until the part I want to fix is isolated.
3. Edit the part until it looks the way I want it to look.
4. For the part I want to mirror, still in Edit mode, hit A to select all
5. Hit Shift+D to duplicate the part. Then, without bumping the mouse
6. Hit S to adjust scaling,
7. Hit Y to mirror about Y (my particular use case)
8. Type -1 and push Enter to mirror the part about Y. This causes the part to flip about the centroid/center of the part, not about origin, so I flip about the XZ plane by continuing on with the following:
9. With the duplicated faces still selected, go to Mesh > Separate > Selection to move the duplicated part to its own mesh.
10. Tab to Object mode
11. Go to Object > Set Origin > Origin to Geometry. Now the origin is at the centroid/center of the part, which is how it was flipped with the S/Y/-1 commands earlier. Now I go to the transform and swap the Y value to the opposite polarity of whatever is there.
12. Now I have the part duplicated and mirrored about the XZ plane, but it's on a separate mesh, so
13. (Still in Object mode) click duplicated part, then click the original, then hit Ctrl+J to join the meshes. Doing it duplicated then original preserves whatever the original origin was.

That's it! What I was doing previously was to set each point in the mesh to the same transform values as the other side of the mesh, but it's easier to just fix one side, delete the other, then duplicate it like this. I've been going back and deleting one side's faces if the vertices went right up to the XZ plane, because I worry the surfaces aren't actually merged on the XZ plane.

Maybe there's some better way to go about cleaning up a mesh like this, but this question went unanswered for a day and I've got work to do, so this is my workflow right now. If someone else has a better suggestion I'm happy to accept that answer.