Toon Shader applies to only half of the model (or is messy)

Question

I have followed tutorials for the Toon Shader. It works for single objects or objects that never had the mirror modifier applied to them, but whenever I join objects, it only applies itself to one half of the combined object.

Even when I join the object into one piece and apply the Toon Shader, the outline is improperly shaped, being distorted and stretched in a very random fashion.

I have tried both individual solidify modifiers for each material and one solidify modifier for the entire object, but nothing has worked.

Is there any way to fix this issue?

Answer 1

This looks like an inverse hull method, so I reckon this has to do with modeling, not shading.

Inverse Hull Method - NPR Wiki

I hope you made the outline shell using a Solidify Modifier, so that it is editable at any time?

For the issues with mirrored objects, I guess this is because you mirrored them by duplicating and scaling by -1? You can do that, but this essecially invertes the mesh, making its normals pointing inside instead of outside. Also, this is a dated method.
Nowadays, we just use the Mirror modifier: more flexible, non-destructive, and no normals issues like this.

To check your normals orientation, you can enable the Face Orientation overlay:

To fix this, ↹ Tab into Edit Mode, select everything with A, and hit ⇧ ShiftN to recalculate the normals. Hopefully, if your mesh is correct, it should set all normal is the right way.

This should fix the mirrored objects issues.

As for the irregularities in the outline, it really depends on the model.

In your Solidify modifier, make sure Even Thickness and High Quality normals are enabled. Then, you can use a vertex group as influence and paint in Weight Paint mode where how thick you need the outline. Thicker outlines are more likely to intersect other mesh parts, so crowded areas like the face usually have smaller thickness:

If you can't reach the result you need this way, I suggest you to save a backup file from there, then apply your Solidify Modifier, and then edit it by hand to better adapt it to your mesh needs.