Blender 2.9.1 Boolean Modifer Union Im a blender newbie and using blender for 3D modelmaking for printing.
I have an issue sometimes when I join two objects. Initially I was using the JOIN operation but now I use the Boolean Union. the 3D Slicers (Cura in my case) doesn't like join. However, I do have a new problem.
Sometimes, not always, when I Boolean Modify and union two objects, the face of one or both objects becomes kind of shaded. In these cases, the slicer always fails.
I've looked at the mesh and duplicate vertices and non of that seems tobe the case.
Can some explain to me:

what is going on?
how to fix this?
is there a better way of doing this?

I have included a pic of a cone and a cube. The cube is affected.
The zappy looking face on the close side indicates a face in the same location. Are you using the Boolean Modifier on the cone with the cube as a separate object? If so, I highly recommend having the two as one mesh object and using the edit mode Boolean operation instead. To fix the situation you have without doing this, apply the modifier on the cone, then delete the cube: The cone’s current modifier-added geometry is identical to but not the same thing as the cube and is overlapping it.