Found a really weird issue while uploading a Blender model to Sketchfab. I've found a work a round and I thought I'd share just in case it helps others. Also to see if anyone can cast any light as to what's going on.
I created a model in Blender and mirrored 3 of the objects (in Object Mode). Everything is fine in Blender but when uploaded to Sketchfab, 2 objects look semi transparent. These objects have the same material as other objects in the model which look fine. Something is going on with the normals on 2 of the 3 flipped objects.
I tried everything including recalculating and flipping the normals on these two objects but nothing works and you cannot get the material to look right in Sketchfab.
My work a round
Do not mirror objects in Object Mode. Go into Edit Mode of the object you want to mirror, select all vertices and apply a mirror modifier. Go back into Object Mode and apply the modifier. Duplicate this object, then go into edit mode for each of these objects and delete one half of the meshes (opposite sides). This gives you the two mirrored objects you wanted and works perfectly in Sketchfab.
Does anyone know what causes this and why 1 of my objects mirrored perfectly but the other 2 went wrong. The only thing I can add is the object that mirrored OK had a lot more vertices than the 2 that didn't.