jserb;
If you see imperfections here is one thing you can try. If you plan on painting the fiberglass, you can add a guide coat (light mist of contrasting paint over the area) this will guide you as you block sand showing you the low spots. (were the paint remains after block sanding) Add bondo (automotive filer) to low spots and sand smooth.
If the fiberglass is over the wood you intend to keep. First take a pencil and holding it at a sharp angle to the hull, lightly rub lines down the hull in opposite direction to your sanding. These become your guide coat. Block sand as before and reveal your low spots. You will fill the low area with resin. Be sure to sand out your pencil lines before adding the resin to raise the low spots so that they will show up under your finish. Sand smooth after cure and finish.
Hope this helps.