Graft it back together and fiberglass it invulnerably strong. This project is good preparation for making a board from scratch.
This repair method uses epoxy resin and microballoon putty. Instead of epoxy you could use polyester resin or vinyl ester resin, with minor changes dictated by the resin instructions. Remember that unlike epoxy, those other resins dissolve styrofoam. This board is urethane foam so it doesn't matter.
Stuff you'll need:
Epoxy resin and the directions to not screw it up.
Fiberglass cloth. I have 8 oz and 4 oz.
Tyvek bunny suit to keep fiberglass dust off your skin.
safety glasses, dust mask, gloves for ditto.
Angle grinder, sanding disk, backing pad.
assorted other abrasive spinning devices, sanding blocks, sandpaper
Step 1: Scrape Off the Wax
If there's any wax on the deck, scrape it down to the deck with a serrated glue applicator.
You can also use a "comb" which is a similar wax texturing tool sold in surfing stores for this purpose.
Then use the straight edge of the glue applicator or any sharpened-credit-card type tool to scrape off as much wax as you can.
Non-surfers may wonder why people wax their boards, and why it looks like it was waxed with candle drippings, not like an ad for floor shine products. The purpose of sufboard wax is to provide a non-slip surface for the feet. So rougher is better.
The wax also gets sand stuck to it. If you surf bare-chested your abraded flesh will get embedded in the wax and your nipples will bleed. You'll realize why "a shirt worn while surfing" is called a "rash guard".
Step 2: Heat Gun and Rag to Remove More Wax
Use a heat gun and clean rag to remove more wax. You'll see the wax sheet out in front of the heat gun when it melts. You can feel when most of the wax is gone, because the rag will slide much more easily over the board.
Then soak a new clean rag with isopropyl alcohol and rub down the board to remove any remaining traces of wax. Wax would prevent the new fiberglass from sticking properly
Step 3: Clean the Wound
Some of the fiberglass skin separated from the foam core. It's possible to glue this back down, but that wouldn't gain us anything since we have to glass the joint anyway.
Cut away the dead skin around the wound. Don't rip up any that you don't have to.
Step 4: Grind Down the Deck Around the Break
Wear a respirator, glasses, gloves, and a bunny suit (disposable coveralls) or you'll be itchy and prickly from fiberglass, blow your nose all night, and get a sore throat. (like I did)
I used a disk sander with a 50grit disk and a backing pad to feather down the fiberglass skin all around the break. You need to make room for the new fiberglass or you'll have a bulge over the break. That would be fine on the deck actually, but you wouldn't want it on the bottom of the board.
Use a light touch and good concentration. This tool cuts fast.
Step 5: Mix Micro-Balloon Putty
Micro-Balloons are tiny hollow spheres of glass or plastic. Mix them with epoxy to make very lightweight putty to fill large holes, or in this case to glue two surfaces that don't mate very well.
Mix the two parts of the epoxy together first. Follow the directions. If you use "ketchup pumps" a.k.a. "metering pumps" be very careful with them. They can gulp air when you're not looking and give you short shots. If you do your measuring with a scale, remember that the two parts have different densities. A 2:1 mix by weight is not exactly the 2:1 mix by volume the manufacturer recommends.
I cut a slot in the lid of a yogurt tub to mix the putty. The microballoons are very light, and a puff of wind makes them go everywhere. The slot keeps most of them from blowing away.
The wipeout that broke this board distorted the foam very badly. The two halves don't mate properly. Or rather when the halves mate well the board is very crooked. So lots of putty is needed.
Step 6: Slobber It On
Smear the microballoon putty all over the mating surfaces.
Step 7: Clamp and Fixture the Board
I splinted the board together with scrapwood, rope, plastic sheets, innertube, shims, etc. etc.
I eyeballed it and felt it to make sure the board isn't kinked. It's a challenge to get all that right while the sticky putty slowly sags and runs away from where you want it. After the board was clamped I squeegeed the putty to where I wanted it.
In addition to the major break, there's also a big wrinkle that will need to be ground out, puttied and glassed.
Step 8: Fix Everything Else
While I had plenty of epoxy putty dripping on everything and a bloodstream full of caffeine,
I glued and clamped some other stuff. A kayak paddle with a split blade, a broken centerboard, etc.
It was a nice hot day and the epoxy set up like cheese just as I got the last thing clamped.
Step 9: Patch a Ding in Another Board
The board needed some more putty to fill the crack after the splints came off. There was some putty left over from that operation.
So I patched a big ding in a windsurfer with the left-over putty.
First I ground down all around the dent.
Then mostly filled it with microballoon putty.
Then put a couple of layers of fiberglass cloth over it.
Then painted that with epoxy until it went clear and looked good. When that was hard I
sanded that smooth and painted it white to match the board.
The ding was on the deck, so I didn't make it very smooth. Rough is better for traction.
See the sliding track slot in the deck? That's for the mast base. I didn't have the base to mate with this track so I butchered it. I pulled the sliding guts out of the slot and replaced them with an aluminum block with an 8mm threaded hole. Then I could use a standard shortboard base with this board. I went windsurfing and decided this was the perfect board for me. It was such a great session I forgot to tie the board to my truck. It blew off in the parking lot and I never saw it again. I forgot to put my contact info on the board.
Step 10: Plane the Joint
While the putty is still like cheese you can shave it down using a 3M "surform" tool which is a lot like a cheese grater. That's how bodymen (panel beaters) do bondo on cars.
After the cheese phase it gets harder like crackers. Wait for it to get hard like fiberglass or it'll just clog your sandpaper.
I tried a bunch of tools to see what was best for flattening the joint. First , I used a coarse sheet on my pneumatic "planer" to flatten the area of the joint and fair it with the board. This tool is an oscillating sander. If you're a boat person you may recognize this process as powered "long boarding" which is fairing an area with a piece of sandpaper on a long board.
I wanted something that cut faster, so next I sanded with my dual-action sander in disk-sander (non orbital) mode. The backing pad disintegrated so I switched to an angle grinder with a sanding disk.
That cuts faster still and can make a decent job fairing if the backing pad is good.
That's mostly what I use now. Anyone want to buy a nice pneumatic planer?
Step 11: Glass the Joint
Mix your epoxy. Time/temperature/humidity affect curing time. Every type is a little different. The stuff I have now is too slow unless it's really hot out. So I let it sit in the cup for 10-15 minutes until it feels just faintly warm. That way I'm sure it will cure pretty fast. While that's happening I'm cutting the fiberglass cloth that will go over the joint. I've got some 8oz cloth and some 4oz cloth to go over that.
The 4 oz cloth sucks everything flat and will let me sand without cutting the weave of the heavy cloth underneath. SCANDAL!! many surfboards are made weak by sanding the cloth. Possibly this board broke for that reason. Actually this board looks like it got stomped by a gorilla while propped on sawhorses.
I pieced this layout from odd scraps because it's a deck and is supposed to be rough. My biggest piece of 4 oz went on top. Exposed edges of cloth stand up and make ridges when soaked with resin. So put your small pieces under your bigger pieces so you only get one ragged edge.
Wrap the glass around the rails (edges) of the board. Tape the glass to the underside to hold it tight and make sure it doesn't fall down.
To wrap a tight bend without the cloth lifting, cut the cloth at a 45 DEGREE ANGLE TO THE WEAVE.
Then the fibers cross the bend at gentle angles and the cloth will lay onto a much tighter bend. Try it and see. I wish I'd known about this a long time ago.
Step 12: Attack the Wrinkle
Sand down the wrinkly fiberglass where the board got bent and cracked. Cut the dead skin from around the wound. Putty it up. Sand it down. Glass it. This is getting to be a routine.
Step 13: Curing Oven
I'm getting sick of waiting for epoxy to cure. If it takes a day for epoxy to cure and the project has 25 steps and you randomly miss days due to job, family, and other distractions, how long will the project take? Forever damn it!! Joshua Slocum (author of "Sailing Alone Around the World") built a yacht 100 years ago with a hammer, saw, and nails in less time than it takes us now with robots and laser beams.
To make the epoxy cure faster I built a curing oven from a big plywood packing crate. I threw a pair of chairs on their sides inside this box to serve as a shelf to hold the surfboard up. Two hot air guns blow hot air into the box, one from each side. Later I added a third. I clamped the lid on with homemade clamps and taped the lid to stop air leaks. To check the temperature I put my hand on the lid. It gets up to about 100 degrees inside.
Step 14: Undercatalyzed!
The curing oven didn't work on the last batch of putty. It's not getting hard.
No amount of baking in the curing oven does the trick.
I check the epoxy metering pumps "ketchup pumps" and see that the hardener pump only springs up part way when you pump it. Damn. I've been getting short shots of hardener.
How do you remove this crap? By trying. This big sanding disk got all caked with crap. Then I cleaned the sanding disk by grinding on that chunk of steel square tubing in the background.
Then back to the surfboard, trying to knock the crap off the board. And repeat.
It was kind of like smearing clay around with a big eraser.
Eventually the board looked like it had been abused by mechanics and the bad putty was mostly off it.
Step 15: Massively Paralyzed Processing
Since there's a big wait after each step, might as well fix another board at the same time.
Here's a broken kiteboard. I patched it with an "inverted pyramid" stack of cloth over the break. 2 layers of 8 oz fiberglass cloth and a layer of 4 oz on top of that. Each piece of cloth is larger than the piece under it. That way the ragged edges don't stand up. Here it is soaked clear with epoxy. Already very smooth. It's not going to break in that spot again.
You can't even see the edges of the cloth layers underneath the 4 oz. That's the beauty of putting the big thin layer of cloth on top. It keeps those ragged edges from standing up like a terrain model.
Step 16: Glass the Bottom
Back to the big board. A layer of 8 oz glass and a layer of 4 oz over that. I wait 10 minutes for the resin to react after mixing it up. It starts barely warming up before I paint it on the board, lay the cloth, tape it around the rails and saturate it. On a hot day I wouldn't wait, I'd mix, paint it on, and hurry, because it's likely to "kick off" in the cup, get really hot, smoke, get hard in a big lump, and melt the cup it's in.
Step 17: Hack Off the "Grass Skirt"
Cut off the extra cloth before sanding the new glass smooth.
Put on your bunny suit, safety glasses, gloves, dust mask for the next steps.
Step 18: Feather It In
These little abrasive disks from the autobody supply store are great. They screw into a little rubber backing pad on a die grinder. Sand the edge of the new glass until it's so smooth it won't cut your arteries.
Flip the board over and use the orbital sander to smooth anything that grossly sticks up from the rest of the new glass. Don't cut the fibers of the cloth under the 4 oz. Don't even cut that if you're really an artisan.
Step 19: Finish the Board
I oven cured the glass layup in a hurry. The epoxy pot still had some flow to it when the board came out. It was too thick to use the paintbrush, so I cut a squeegee from a sheet of plastic. I squeegeed the epoxy all over the newly sanded glass and put it back in the oven. Notice the "return address" contact info on the board. After all this work I don't want to lose the board.
It's done! Go surfing!
What if you want it perfect? Then you'll sand it again and epoxy it again and repeat and repeat until a car falls from the sky and crushes you. Everyone will say what a tragedy it was that you wasted your life turning perfectly good epoxy into noxious dust over and over again beyond all reason.