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
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
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
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
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
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 7: Clamp and Fixture the Board
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
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
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
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
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
Step 13: Curing Oven
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!
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
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
Step 17: Hack off the "Grass Skirt"
Put on your bunny suit, safety glasses, gloves, dust mask for the next steps.
Step 18: Feather it in
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
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.