1. In Constructor, switch to vertex selection mode.
2. Select all of the vertices.
3. In the prefs tab of the properties panel, set the grid size to a very small negative power of two, like 0.0078125.
4. In the modify tab of the tools panel, click the quantize button.
I'm not sure about this, but some vertices that used to be in the exact same spot might be spread apart slightly. If that happens, you'll want to manually move a few of them so there are no cracks (or overlaps). Just make sure that there are no red lines; those are indicators that the faces with them won't export properly. A single face must have all of its vertices on one plane, and brushes can't be concave.