How to fix Cracks/Gouges on a Canoe
Before anything, you have to determine what type of crack you are going to fix, it will determine the amount of work needed and what type of material you will need to use.Superficial Scratch / Crack / Gouges on the gelcoat:
I did not see that rock! That happens. It mostly cosmetic and as long as you do not see the fiberglass under, it's easily fixed with minimal effort.
Oops! Crack
While carefully carrying your Canoe, you somehow let it fell, leading to mostly 2 types of cracks.
1) Driveway landing. The Impact Zone was flat and it created a long(er), non perforating crack
2) Sharp rock landing: The inpact zone was hard, small, and sharp. It created a small(er) perforating crack.
Age Cracks
Your Dad spent his youth summers on this Canoe, then his summer interest switched to girls (ahem..) That was 25 years ago. You have now realized that this Canoe can carry a ton of beer and few girls too.
As cops can't reach that small island on the lake, it's now your Party Headquarters.... But the Canoe now need some TLC.
Star Cracks
Often caused by small impacts, the pattern of the crack is in Star Shape. Often, the center will have to be removed as the gel coat has cracked in small bits.
Freeze crack: Your Canoe had a crack, and was left outside, uncovered in winter. Most likely, water seeped in and froze, then thawed and froze again ...Each time, the ice expands, leading the crack to expand too...
Some of those cracks looks tiny but they run parallel between the gelcoat and the fiberglass. Each Freeze cycle will "lift" the gelcoat from the fiberglass and a Nickel sized crack can lead to a 4"x4" repair patch.
Inspection:
Believe it or not, there is nothing more frustrating than to start a repair and realize in the middle that "that little crack" is in fact a huge repair, so inspect thoroughly then make a prognosis: You'll save few trips to the Hardware Store.
How to inspect:
is it perforating or not? Leaking or Not?
Look a the following tricks to inspect the hull and determine if and where it leaks, or not.
Dry inspection
Magnifying glass
Well, pretty explicit.. look closer :)
Poke
Gently press with a finger and see if the coat "gives". If yes , it could be an "undercrack" between the gelcoat and the fiberglass (Freeze Crack), or a factory default. There is an example below of a huge factory default.
This will lead to a mandatory fix with Bondo and 3 layers of resin.
Bondo is messy to work with and needs sanding, but this the -almost- only choice you have when you are in need of a filler. The second solution would be layers after layers of fiberglass cloth and resin: It will take you a while.
Light
Run a flashlight against the hull and look for light.
Wet inspection Use water to see if it is either "absorbed" or leaking. Before filling the canoe with water, please consider the weight of water: 8.35lb/Gallon or 1kg/liter. Don't fill up your Canoe up to the rim and always use the minimal amount of water.
If you have a Shop Vac, most of them can be used as a blower: Place the tube against the hull and use soapy water: You'll then see a bubble forming.
Mechanical
Well... you are going to fix that crack/gouge aren't you? So grind it with a Dremel
, and you'll have a quick answer to your question...The Works
Before you even start thinking of any kind of repair, A word (or two) of caution:
Fiberglass, Resins, Catalyst, Epoxy, etc are Nasty Mistresses
I can not stress enough that you have to wear -at all time- protective gear:
Eye Protection, Gloves, Skin and Respiratory protection.
Children must be kept away at all time. Use Duct tape if necessary.
Resins and catalyst can cause skin burns!
Sanding/grinding fiberglass can free -airborn- micrometric particle of glass and resin(s)
"Long sleeves" are also highly recommended
No open food container (No uncovered coffee cup, etc etc ... )
- The use of a "Dust mask" is NOT recommended. Those things are good when "cleaning" your garage, or mowing, but are worthless and potentially dangerous with fiberglass: They get loose after a minute and most likely do not provide a good airtight fit on your face.
Furthermore, a Dust Mask will most likely fog your safety glasses: You can't see a damn thing, you're coughing, hitching, and trying to remove your goggle with resin sticky fingers... and then, when you'll explain what happened to your dermatologist, optician & ophthalmologist, you'll remember that you ignored this kind warning.
The use of a Half-Mask
/ Respirator is HIGHLY recommended: The 3M is so good that I can not even smell acetone, lacquer, and as a matter of TMI, my own farts. It's also comfortable, allows to breath easily and speak! (instead of the usual Muhmumu-Muh-Huh). It's less than $30 and is worth every penny. 
3M Half-Mask
For those wondering, P95 means that it filters 95% of all airborne particles, P100 filters 99.7% (HEPA)
3M P100 Particulate Filter
Other gear and equipment that you will most likely need:
Nitrile Gloves
and/or Latex Gloves
I recommend the nitrile gloves, for the reason that latex gloves are very fragile and you'll go trough them at very high speed. A ripped glove is no fun when you work with resin: By the time you remove the glove(s), clean your hands to remove the resin, dry them, put new gloves, you will most likely look at a semi-set batch of resin. You'll then try to hurry, mess up, bitch and moan, and re-do the job.
buy the nitrile ones: You'll save time and money!
Tip: When working with fiberglass and resin, put two gloves per hand: If you need to touch the resin, and specially when working with fiberglass mat, you'll end up with very sticky fingers covered in resin and fiberglass strings. You can easily remove the dirty-sticky first glove and continue working without interruption: Timing is paramount: Resin set within 15 min, once set, it's set!
To put 2 (or 3) gloves per hand, use a little bit of baby powder between each gloves.
Safety Glasses
Two choices: You either have safety glasses, and wear them, or you hire an eye doctor on site, to be next to you. Having a micrometric shard of glass in your eye will bring you to the Emergency Room, -in pain-
Hardware
The Complete list of what does what is here.
This list is based on a broad type of fixes, you will NOT need "everything"
Everything you always wanted to know about fiberglass
Short & concise. You'll be an expert in 10 minutes or less.
(Portable Flotation Device)
is perfect for the iPhone type. 
, you can find very cheap ones that will save you the day. Being left without propulsion is a not a funny experience.
is an Epoxy Putty that cures underwater in 30 minutes
