All paper tears should be mended before re-attaching a signature to the text block. Various techniques for repairing damaged paper are discussed in earlier posts within the Book Repair Basics category.
Examine the sewing holes on the fold of each folio. If the sewing holes are enlarged or the paper between the holes is damaged, the fold should be repaired before the folio is re-attached to the text block. If the damage is limited to one or two sewing holes or to only part of the fold, repairing that portion of the fold is sufficient.
It is not always necessary to repair each folio of a signature. The center and outside folios often sustain the most damage. Repairing only those two folios is faster and decreases the thickness of repair tissue in the repaired signature.
When most or all of the folios in a signature are badly damaged, each folio should be repaired.
Repair the folds of the folios in the folded position. Paper repaired flat then folded tends to have a very sharp crease while the original signature folds tend to be rounded. Repairing the folios in the folded position helps retain the proper shape and placement of the fold.