Delete Coplanar

Deletes edges between two faces that lie in the same plane, the blue edges in Analyse, so the two faces become one. This tidies the model; it isn't needed to make it solid.

Group: Repair
Fixes: Blue: edges of coplanar faces
Works on: Your selection, or the whole model if nothing is selected, including groups and components nested inside. In a component, every copy changes.
Version: Full

How to use it

Run Analyse and look at the Edges of Coplanar faces (blue) counter.
Select the parts to clean up, or nothing for the whole model.
Click Delete Coplanar.
If the counter went down but isn't 0, click it again. Some models need several passes; stop when the counter no longer changes.

Before and after

Analyse view of a figure with blue coplanar edges; the counter shows 232 edges of coplanar faces — **Before:** 232 edges of coplanar faces (blue). After Delete Coplanar the blue edges are gone.

Limitations and intentional geometry

The edge is deleted even when the two faces have different materials, so the merged face takes one material. If a split is there on purpose (a colour boundary, a texture seam, a line you need to push/pull later), select only the parts where it doesn't matter.
Faces that are only almost in one plane are left alone. Their edges stay blue and stay in the counter. Repair Distorted Faces can help there.
Blue edges don't stop a solid. If you want to keep them, hide them in Analyse Filter.

Check the result

The blue counter shows 0, or stops changing when you run the tool again. The solids you had are still solid (magenta).