Imported models have overlapped/duplicated geometry (triangles) and need manual cleanup to isolate the quad geometry and get rid of the floating (unwelded) triangles.
The quad mesh uv's must be then welded, because even if they are perfectly in place, each quad is cut away from the others as an individual uv-island. (This makes for bad performance in gaming because you have quadruple the vertex count in GPU)
Imported models have overlapped/duplicated geometry (triangles) and need manual cleanup to isolate the quad geometry and get rid of the floating (unwelded) triangles. The quad mesh uv's must be then welded, because even if they are perfectly in place, each quad is cut away from the others as an individual uv-island. (This makes for bad performance in gaming because you have quadruple the vertex count in GPU)
https://www.youtube.com/watch?v=J5wfZKwUaE8&feature=youtu.be&fbclid=IwAR0TfVU5M5sX0hrJk0urnBJJpEn7UPxD8QW5r5542GiDUwQaFl6aWl84pQ4