Surface Edit > Orientation > Unify Surface Orientation

Forces sets of adjacent surfaces to have consistent orientations (all facing the same way).

Modifies a collection of patches so that sets of adjacent surfaces face in the same direction (either outward or inward). This helps to ensure that lighting is correct in renderings, and is also useful when modeling, and offsetting surfaces.

This tool modifies the orientation of surfaces based on topology. It is useful to fix surface orientation on smaller parts (for example, a door handle) where it might be difficult to move the camera around in order to see all the surfaces at the same time.

Construction history is preserved during this operation.

To unify the orientation of a number of surfaces

Choose Surface Edit > Orientation > Unify Surface Orientation.
You are prompted to:

Select input surfaces. When finished, click ‘Classify’.
Pick all the surfaces to be unified, and then click the Classify button in the lower right corner of the active window.
Based on the Topology Distance tolerance value (set in Preferences > Construction Options, under the Tolerances: Topology subtab), the surfaces are grouped into sets of adjacent patches. Each set consists of those surface patches that are within the Topology Distance tolerance. The sets are drawn in different colors to help differentiate them.

An arrow shows the direction of the unified surface orientation on each set. To change the orientation of the set, click the arrow.
When all surface normals are oriented correctly, click the Unify button.
You can continue changing surface orientations, then clicking Unify again, until you select another tool.

Unify Surface Orientation is a continuous function. To finish using this tool, select another tool.