Is Your 3D Print Model Watertight? Part 1: Front and Back Faces


So, you have a SketchUp model you want to send to a 3D printer. To convert from SketchUp’s format to STL, you can use the  STL Extension or the STL feature of the 3D Warehouse.

But before generating an STL, it’s always a good idea to check that your model is watertight, also called “manifold.”

Why Watertight?

Most 3D printing software requires watertightness in order to print correctly. A non-watertight model will generate a problematic STL file when converted.

The obvious meaning of watertightness is that the model should be a single, entirely closed volume that would hold water if filled. So no holes, no internal faces. But less intuitively, your model should also have all surfaces facing the same way.

You probably already know that SketchUp faces have a front side and a back side, and the default front and back colors are different. For watertightness, the front color should always be showing on the outside. This seems simple enough, but it’s actually easy to lose track of what’s facing where, especially when you’ve already painted faces with colors or materials.

(Keep in mind that lack of watertightness is something that can be fixed for you. The 3D Warehouse STL generator – powered by Materialise – checks for watertightness and flips faces when possible, along with fixing other types of problems. There’s also a free service called netfabb which provides similar model optimization. But checking a model yourself takes very little time, and it’s helpful to understand where, and why SketchUp displays front or back faces.)


Monochrome Faces

Here’s a simple example: text created with the 3D Text tool, and a box. Both objects are painted. 3D text is created as a component, so when it’s painted from the outside (without opening the component for editing), the whole thing gets painted, both front and back faces.

3D Printing Watertight Model Monochrome01


The top of the text is moved to the top of the box, so that the letters can be engraved into the box.

3D Printing Watertight Model Monochrome02


After exploding the text component, and removing the front faces from each letter, here’s the model I want to print. It looks fine, but I can’t actually tell if any of the visible faces are back faces, because everything is already painted.  3D Printing Watertight Model Monochrome03

So here’s the very simple way to check: choose View / Face Style / Monochrome. This paints every face in its default front or back color. Gray is the back face color for this model, so I’d end up with a problematic STL file. (Some 3D printing software would be OK printing this, others wouldn’t.)3D Printing Watertight Model Monochrome04


To see your default face colors, open the Styles window. Open the Edit tab, and click the second of the five “cube” icons along the top – this opens the Face settings. In this particular template, white is front, gray is back. 3D Printing Watertight Model Monochrome05


White and gray are not the most useful colors when checking for watertightness, particularly against a white background color. So if you really want your back faces to be obvious, change your default colors. Now I have yellow fronts, purple backs.

3D Printing Watertight Model Monochrome06


Flipping Faces

Switching back faces to front is easy. Right-click on any back face and choose Select / All with same material. 3D Printing Watertight Model Monochrome07


When all back faces are selected, right-click again on any one and choose Reverse Faces. 3D Printing Watertight Model Monochrome08


Now the model is ready to convert to STL. 3D Printing Watertight Model Monochrome09

If you plan to continue working on your model, remember to switch back to Face Style / Shaded with Textures. There have been times I’ve forgotten to do this, and it’s quite frustrating when you’re trying to paint monochrome faces – the new materials or colors don’t display.

In Part 2 of this series, I’ll show an extension which will help you fix all watertightness issues – back faces, extra faces, and holes.


About Bonnie Roskes

Bonnie Roskes has been writing tutorial-style projects on 3D modeling software, primarily SketchUp, since 2001. Her website, offers a wide variety of learning materials for all ages, from kids in grade school through design professionals. Her materials cover general 3D design, 3D printing, geometry, interior design, geo-modeling, and more, and future books are in the works. Read more about Bonnie.


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