How to build small
“bridges” across gaps within a mesh, so that Mesh > Mesh Cleanup > Mesh Hole Fill
can be used to fill up the
remaining holes.

Build a bridge across a gap in a mesh
.
The mesh boundaries are highlighted in green.

first to combine them.

Locators indicate the location of the endpoints. The edges that will be used in the bridge are highlighted.
A “bridge” made up of two triangles is built between the endpoints.

.