I don't know about you, but I used to hear about fiberglass and surfboard repair and feel pretty intimidated. Surfboards are beautiful, and their creators are craftsmen and artists of the highest order. It turns out, though, that fixing dings and cracks is something that is well within reach of anyone with some sandpaper and patience. And it doesn't stop there! Fiberglass and resin are pretty amazing work materials, and I'm sure you crafty folk will find a thousand more uses for this skill once you've got it in your toolkit.
So, lets do this!
This may sound like a long list, but it's all cheap, cheap stuff (minus the optional electric sander and dremel)
-Sandpaper in a good variety of grits, course to very fine
-An electric palm sander is nice but not necessary
-Ditto for a rotary cutting tool like a dremel (don't think I used one on this board)
-A sharp blade like a utility knife with replacement blades
-A dozen 3oz graduated medicine cups (sold wherever resin is sold)
-A dozen popsicle sticks
-Roll of masking tape
-Box of disposable gloves
-A dozen disposable cheap paint brushes, also sold where you buy resin
This is the barrier. I haven't found polyester resin sold in real small volumes. 1 quart can repair dozens of serious dings, and the stuff starts to expire after 6 months, so I just wish I didn't have to waste my money on so much of it. Ah well though, if you find a source for smaller volumes, please let me know in the comments!
Onward then, you'll need:
-A small amount of clear-curing polyester resin
-A much smaller amount of matching catalyst
-Some kind of filler material (whatever they sell wherever you get your resin)
-A sheet of fiberglass, at least 4x the size of your ding if you're an amazing craftsman, probably 10x if you're like me. Don't worry, it's cheap at those sizes.
Step 1: Clean the Board, Find the Dings
This is an important step because you really want to get the whole job done at once. Patching five dings is really not much more work than patching three, but patching three and then discovering you have two more a week later is a real bummer.
So put your head down, scrape the wax clean and get this right. By the way, doing things 'right' is going to be a major theme of this instructable. This is not a hard job, but you need to put the front-end work in if you want it to last. And if you do, it will. The photos from this instructable are 8 years old, and so far all the patches have held up flawlessly. So take your time, crack a tasty beverage, and enjoy becoming a part of your board.
Step 2: Prep the Patches
Ok, it's cutting time.
What you want to do here is go back to all those cracks and dings you ID'd in the last step and with either a utility knife or a dremel eliminate all the damaged areas. Any cracked or fraying fiberglass, and any gross black foam underneath. You should be looking at crisp, clean glass lines and milky white or at least toasty brown foam.
I'll say it again, put the time in now to get it right. Have another beverage, put the board away somewhere dry for a day if you have to, but make sure you end with a product you'd be proud to post pictures of before you move on. You'll thank yourself when this things still floating under your grandkids.
Then, when you're done with that, take your sandpaper and put a nice 45 degree bevel on everything. You want the new glass to have some horizontal surface area to grab onto.
Step 3: Fill the Holes
I enjoy this step.
Now you need to backfill all those cavities you created. One of the nice things about this step is that you don't have to be very careful at all.
First, you build little basins around your holes. With small holes, you can use masking tape. For large forms, like the tail in the project I've pictured (looks like I did end up cutting away all that dead foam, go me!), you might want to use cardboard (and gorilla tape, boy I love gorilla tape). You'd like your basins to be relatively tight, you don't want to waste materials, but you don't have to be a perfectionist here at all.
Then, working in 3oz batches, mix equal parts resin and filler plus catalyst according to instruction (based on the volume of resin, not the total volume you're mixing, by the way) and start pouring into all your molds until you're certain you've at least filled the holes. Overflow is not a problem (just protect the ground beneath you with a tarp or something).
Step 4: Rough Carve
Then pull the tape and voila! You're left with these cool little globs.
These are also really satisfying if you catch them just right. The rate of drying varies wildly depending on how much catalyst is in your mix and the temp/humidity, but if you can catch it a bit after it's taken shape, maybe 10 minutes after the pour, and start cutting and sanding it then, it shapes so nicely. I start with the knife and then progress to sandpaper when the knife stops working. You can attack it the next day if you like, but you'll have to grind a lot harder with your sandpaper, so I'd try to catch it as soon as it feels rigid under the tape.
Anyway, the goal in this step is to bring your filler in line with the foam. You want to sand until you're just a millimeter below the ding. This is where the real artistry comes in, this is the make-or break step for getting your board back in the right shape. It's not hard though. Just watch the lines and match them. You're just putting the last little dash in an otherwise beautiful line. Believe in yourself, take your time and watch it come together. Then give it just the slightest little oversand all around the patch to make room for the glass.
And don't worry, if you screw up, you can just patch again. But you won't screw up.
A quick edit here. It was pointed out in a comment below that I failed to point out that one should always wear respiratory protection when sanding, doubly so when dealing with fiberglass. Likewise, gloves and long sleeves are good precautions to take so that they can be removed after work and isolate your living space from this unhealthy dust.
Step 5: Glassing!
We're finally gonna get that stick back to water-tight!
This step is really easy too.
All you do is lay fiberglass over your patch, give it maybe an inch of overlay all around, then mix some resin and catalyst and brush it on. Wherever you brush, the glass will become instantly clear and... that's it!
Again, work in batches. This time the resin will become hard really quickly. So lay glass, then mix resin, then brush. Don't brush the whole inch of overlay. The excess glass falls right off when you sand around the perimeter. You can do several patches with each mix if you're ambitious, or you can work small, slow and carefully. I tend to do the first, but it's bitten me once or twice, so I'll recommend the second.
Give your glass an hour or so to dry, then sand it smooth and add another layer. Then one more.
There's a special goop that you're supposed to add to your top layer, something about curing correctly, and feel free to ask around about that, but I've never used it and never had a problem. Shows what I know, huh?
Another tip that was suggested in the comments below: pre-brushing the resin and then applying the fiber to that may help avoid bubbles. Sounds worth a shot!
Step 6: Polish
And that's it! Your board is now officially water-tight and ready for action! This last step is all about aestetics. Which matter if you want them to matter. All you do is grab some smoother sand paper and rub till you're happy. The board will get exactly as pretty as you care to make it. I've got some I gave 110% to and some I gave 90% to. They all glide the same, I'm just more inclined to show off the former ones.
I hope I didn't make this sound intimidating. It's not. It's easy. It just takes time and will. You can have exactly the quality of board you want to have. And you'll appreciate your own work every time you paddle out.
See you out there!