Apply Multires modifier to some faces but not others

I am working on a C-130 Hercules project to print on a 3D printer. I started with a .obj file that is intended for use in a video game.




The first photo shows the file in its original condition. When this is printed the individual faces are obvious in the plastic and require lots of sanding. The second photo shows 2 levels of subdivide applied. It is nice and smooth but the window detail is lost.

Is there a way to keep the window detail and smooth the “skin” of the plane’s nose?

I think it’s simpler to use Mesh / Faces / Shade Smooth instead of subdividing. Start with your original imported mesh. In edit mode, select a bunch of faces and do Mesh / Faces / Shade Smooth (or control-F / Shade Smooth). When you render the image, the edges between those faces will be smoothed.

You may need to repeat the operation several times around the mesh, so that you specifically avoid smoothing any edge that’s supposed to remain sharp.