Fibers can only protrude from forms where there is a joint. They can not protrude in the middle of a form. This can be minimized if the joints are caulked before concrete placement. However, it is not always possible to calk every joint. The number of protruding fibers is a function of the precision of the joints and the fiber dosage.
Wider joints will catch more fibers than tighter joints. After the formwork is removed, the fibers can be quickly knocked down with a hand sanding block or a small angle grinder.