A question I'll often get is 'How do I make a section cut look like it cut through a solid timber?' (as SketchUp models with faces, all objects are hollow)

The answer was to create a group from the section slice, then manually edit the group and create faces where solids should be - usually connecting a few lines would fill things in.

There is a plug in available to automate this task - I haven't used it yet - but it was recently featured on the Fine Woodworking SketchUp blog.

You can read about it here.

There is a link to the most recent plugin - but it can be found here.