Gideon,
Sorry I missed your question,”What did you do to get the rough side of the fiberglass so smooth?
Mostly ( metal roller) rolled on polyester resin on chopped strand mat , mainly to repair a lot of spider cracks , tears , holes . After sanding it looked good so I continued on . The small indents were filled with icing . Sealed with sealer, Primed and painted.
The thickness is still about 1/8 range.
Another benefit of smoothing is the fibers are less moisture entering the glass ,freezing and causing fissures. It also smells cleaner and holds less dirt. Mostly I was tired of getting fiberglass cuts from the sharp strands.
Dave