Yes. Seacast™ is formulated to bond to fiberglass. It will bond to small amounts of pre-existing wood that may remain after extraction, but the bond to fiberglass will be weakened and the remaining wood will continue to rot.
Do I have to fix cracks and holes before I pour Seacast™?
Yes. Seacast™ will leak thru any openings in your fiberglass. You can use duct tape (criss-cross) to cover small holes. After Seacast™ sets you have to ensure that the Seacast™ is flush with the inner side of the skin.