In some objects, for example in windows glasses, I find that if I invert one face (because is on grey colour), the oposite faces gets inversion too, so that the problem that is solved in the first face is created y the oposite one.
Is there any option for invert all the faces of an object in the correct way?
Tank you very much.
It all depends on the model, uploading it here will allow someone to give a better answer. If the glass does not have any thickness then one side of it will always be blue.
You can right click on a white face and choose “orient faces” from the context menu. If that doesn’t fix things sometimes that can be because there are internal faces inside and SketchUp can’t fathom what should be which way round. Removing the internal faces often solves that problem. Again, for a better answer upload the SKP.
It sounds like you are trying to make both sides of a single face front faces. That’s not possible. You would need to give the window thickness so you can have front faces on the inside of the window and the outside.
Keep in mind that faces in SketchUp have no thickness. The face only represents an outer surface. In the case of your windows, the glass does have thickness so you would model it that way.