I can't find any tools within the UV-editor (like a mirror modifier) to allow mirroring. Can you somehow define 2 area's being split so that one mirror's the other? Or Should i use an external program like Krita/Gimp for the mirroring part?
I did found out how can with 3D-painting with (Texturepaint -> Tools -> Symmetry). And how to combine this to paint specific parts on the object. But when testing this out on Suzanne it feels less efficiënt compared to painting in the UV-editor.
3D paint mirror-way: (Go to Edit mode -> Select couple of faces -> Select -> Mirror (with "Extend" checkt) -> Shift+H -> Texturepaint -> (left from Layers) "Face Selection Masking for Painting").
Nevermind, i just discovered the mirror option so wich enables overlapping op symmetrical islands :D. Still don't know how alter the visibilaty of lines of the mesh in Paint mode of the UV-editor.
Well, the only 2 things i could make up is not-using the checkers-UV and using a alfa value on the brush (UV-editor -> paintmode -> Tools -> Overlay -> Curve (open the eye and select the brush tool). I expected there would be an option in blender to change the default gray colour. Gray on a half gray checker texture isn't ideal.
While writing this comment i found out that you can change the gray colour in the User Preferences. Problem solved :).
