What kind of tools do you have? Fiberglass repair is pretty easy, but it takes a couple of things to do it right.
Supplies:
You're going to need something to clean up the area, and grind down to into undamaged fiberglass to give your repair something solid to bond to. an angle grinder with some 24 grit sanding disks is ideal, but anything that can remove the damaged stuff and feather out through the paint/gelcoat a few inches out around the area will do. You want to do this right before you actually make the repair, Don't grind the week before and let it sit.
You want a quality
unwaxed resin. Most of the over the counter stuff (Bondo, etc.)contains wax, it must be ground down between layers layers so the next layer sticks. With an unwaxed resin you can keep going as soon as it sets up enough to keep things from running and sagging. A polyester resin will do just fine, odds are it's what your boat was made from in the first place. You can get additional adhesion for vinyl ester or epoxy resins, but the price goes up accordingly and abrasion resistence will not be any better. I like to grab some extra hardener (MEKP) when I order. Sometimes it's necessary to use a little more in cooler weather.
Chopped strand mat (CSM) will be the glass you use to build up the repair. You can layer in glass cloth too if you wish, but given the size of your repair area you won't need that.
I purchased most all my supplies from US Composites.
Other assorted stuff you will need are mixing cups, something to measure resin and hardener, cheap disposable brushes, disposable gloves, a dust mask/respirater and eye protection for grinding and sanding.
The Process :
Grind or sand the repair area to to expose the undamaged glass and feather the edges out. This will give your repair something solid to bond to. This is the worst part, wear your mask and eye protection, a cheap painters suit will keep most of the itchys off you, baby powder on your skin ahead of time really helps too. Vacuum up the mess and wipe the repair area with acetone to be sure it's clean. The better you clean up now the less that stuff will make you itch later.
Next prepare you CSM, Tear it into strips the right size for your repair. Start small directly over the hole and keep increasing in size for as many layers you will need to bring you back up to the original hull thickness. Once you mix your resin there is no time for this.
For the actual repair, it's easiest to flip the hull so your in comfortable position to work and gravity helps keep everything in place for you. Mix your resin in small batches to minimize waste. Nothing is worse than having a cup of start to warm up and gel before you are done. Once it starts to thicken, throw it out, you can't make a quality repair with resin that's turning to Jello on you. Be sure to keep a box of gloves and an garbage can handy. Fiberglass is sticky and messy, it's much easier to just toss it in a garbage can than unstick it from everything around you when you are done.
Hold your strip of CSM over the area and use a brush to stipple the resin into it. Once it wets out ( it will be translucent), it should stick nicely. You goal is to use the minimum amount of resin necessary, the strength is in the glass not the resin. Air bubbles are weak points, You want your glass completely laminated to the layer below. I would let your first layer or two begin to cure before continuing. That will help support the rest of your layers and prevent air bubbles from rising from the backside. From that point on, just keep layering on the glass and and wetting it out with mixed resin the same way until you have build it up to the desired thickness. Laminating your layers wet on wet will produce the strongest bond between them. Strong is good.
Finishing:
Once your repair cures , you're ready to fish. If you want to make it look pretty, you can fill, Sand, and paint. When ordering your supplies, if you get glass balloons, you can mix them with your resin to make a filler. Keep in mind, there is no structural strength in filler and you want to keep it's thickness to a minimum. Regular auto body filler is not recommended below the waterline, it has a habit of absorbing water failing when used there.
Good Luck!
