I don’t know why, but lately I’m diving deep into understanding how my 3d programs work. My modeling skills are pitiful, but digital art allows me to see my artwork better in a way my eyes cannot. The side effect seems to be the constant reworking of artwork without much initial progress.

Here’re some observations for anyone learning Substance Painter detailing why the program may not be accepting your model (it either opens a blank page or the baked surfaces show up as black or gray):

Your object may contain isolated points or shapes with only two vertices.
In LightWave Modeler, press lowercase W to open the Statistics window and select these problematic vertices.

The object may also contain non-manifold polygons—geometry that overlaps itself, has duplicated faces, or includes edges shared by three or more polygons, breaking what should be a closed volume.
To fix this, press lowercase M and use Automatic Mode to merge vertices that occupy the same space. Then press Shift + I to unify polygons that overlap or disrupt the volume.

You can also try the Mesh Repair command under the Details tab (in the Polygons section) to locate non-manifold geometry if you plan to remodel parts of the object. On its own, this tool can behave unpredictably—it often splits edges and isolates geometry you might want to keep—but it does a good job of highlighting problem areas. The inner workings are a bit opaque to me, but it may still be useful in your case.

Finally, the object may include non-planar polygons.
In LightWave, four-sided polygons must be perfectly flat, regardless of their orientation. My admittedly sloppy workaround is to use the Statistics window (lowercase W) to select non-planar polygons and then triangulate them with Shift + T. This can sometimes result in too many triangles converging on a single point, but so far it hasn’t caused me any issues.