My second kayak build was put on hold by my wife's cancer treatment. She is doing well now, and so I'm back at it and determined to finish this project. The hull is done, and the deck top is fiberglassed.  I'm getting ready to fiberglass the undersurface of the deck, but there are some significant defects that cannot be sanded out. What can I use for fairing under the fiberglass? I greatly appreciate help with this...