It must be symmetrical or be posed in
a symmetrical manner (in the case of a character model).
It must have a new or existing paint
layer to paint on. For more information, see
Create a new paint layer.
You can paint on both
sides of a model as follows:
If the model is positioned exactly at
the X, Y, Z origin (0,0,0) in the 3D View,
select the 3D Paint brush, and in the Properties window,
select either X, Y, or Z (depending
on which axis you want your paint strokes to be mirrored across)
from the Mirror drop-down menu.
If the model is not positioned exactly
at the X, Y, Z, origin in the 3D View, select
the 3D Paint brush, and in the Properties window,
in the Mirror option, select either Local
X, Local Y, or Local
Z (depending on which axis you want your paint strokes
to be mirrored across) from the Mirror drop-down menu.