Another Welcome! Julie, I’ve seen guys that can’t successfully remove a screw (my dad was one) and worked with ladies who could dial in a 120 foot long tool. It just comes from what we were exposed to and took an interest in as we get older. The smartest thing you’ve done with is ask for help. Where you need a visual, Andy at
http://blip.tv/boatworks-today/different-types-of-fiberglass-repair-6021211 is superb. There are other YouTube blogs that demonstrate how to make boat repairs.
From my experience, the first thing would be to establish the limit of the void and any hidden extensions. This is relatively easy to do using anything to tap with from a very small hammer, to a pocket knife. When you tap on a place that’s “solid”, that is there’s no separation in the laminating, you will get a sharp return. You can test the sound by tapping on you table. If the return is muffled, there’s an issue. The most common in the area you are working would be “soft wood”. That a nice way of saying rotted. It could also be the result of a past repair or a construction void.
I keep four power tools aboard. A 20v driver, a 20v drill (both Black and Decker), a small shop vac and an oscillating tool. Depending on the extent of the void plus the suspect surrounding area, a simple clean and fill may do the job, Personally I like “Thickso” which is thickened polyester resin. Clean up the hole, lay in the Thickso, then cover with Duck Tape. The suggestions of practicing with the material is spot on. The advantage of Thickso or your own mixture of polyester resin is how quickly it will “kick”. A slow to kick material is an obvious issue with vertical surfaces. If the area sounds like it’s, say 6” x 3”, then there are other options. First the area has to be dry on the inside. Iiwm, I’d get out the oscillating tool and if I could get at it from the back side, cut the fiberglass and layer of reinforcement (plywood) but not cut the outer fiberglass. Next I’d peel the cut area away from the outer layer of fiberglass and then clean and dry the cavity. To backfill, a piece of plywood cut to fit and soaked in a catilized 50/50 mix of resin and acetone would be pressed into or covered with a “buttering” (smeared layer of bonding compound) of Thickso. Push the plywood plug in firmly and use something to hold it in place. After eight hours or so, sand everything flat with the plug are depressed so you can butter the fiberglass layer/piece you cut out and insert it in the depression getting it as close to its original position as you can. Clean up the squeezed out Thickso, and finish the repair based on Andy’s demonstration (the boatwerks guy).
For the time being, as you are deciding how big the area is and your repair plan, you can cover the opening with Duct Tape, I prefer Gorilla Tape.
I too would welcome any questions via private message.