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At the fiberglass supply shops it is called peel ply. It produces a totally different effect than the mylar I posted, it leaves a flat, textured surface that does not need any sanding to go on with the next layup. The mylar leaves a mirror-like finish, for that last layer before paint. Only need a bit of sanding to provide "tooth" for the paint to stick to.
The only thing to beware of is any kind of coating it may have from a regular fabric store. That coating, if present, would mess with the bond of a successive layer.
Peel ply is useful when you lay up an area, then plan on bonding a wall or bulkhead to that surface later. Leave the peel ply on to keep it clean while you work on other things.