How I Made a Carbon Fiber Acoustic Guitar

The 3D models were provided for me by the instructors, so I can’t comment on the process of making them. If I ever pursue this type of modeling further, I may update this step. I’ve included the 3D model and original plans that this guitar was based on — it was modeled after a 1940 Gibson L-00. The sides of the guitar were rounded like an Ovation style guitar, because carbon fiber doesn’t bend well around sharp corners.

I chose to use the stock design provided by the instructors, but others in the workshop chose to customize their instruments — adding cutouts, changing the size and scale of the body, etc. One of the advantages of working with carbon fiber is the ability to make shapes that would be difficult to achieve with wood.

Once the 3D model was created, it was transferred to Autodesk’s free 123D Make program, which is capable of slicing the model into layers to make a cardboard mold.

1. Open 123D Make. Use the button on the lefthand side to import a 3D model, open the Guitar.stl file.
2. Now the model should be displayed in the center of the window. On the left there should now be an option for “construction techniques.” Select “stacked slices,” this will create layers of vertically stacked slices, which is perfect for our cardboard construction.
3. Next, you need to set the dimensions for the object being made, and the material it’s being made from. The program automatically sliced the model into 0.177″ slices, which created 56 parts spread over 23 sheets of cardboard.
4. • My model size was about 935mm (36″) long × 358mm (14″) wide × 111mm (4.37″).
   • My material size was 24″ long × 18″ wide × 0.177″ thick pieces of cardboard.
5. Inspect the model and sheets for any errors. If everything looks fine, go to the bottom of the lefthand menu and select “get plans.”
6. Select the desired format at the bottom of the window. Exporting .EPS files will allow the sheets to be opened in Illustrator or CorelDraw, where they can be sent to the laser cutter.

For those without personal access to a laser cutter, you can still use the model by sending either the 3D model or the exported files from 123D Make to outside fabrication services that can create and ship a mold to you.

The mold files were exported from 123D Make into Adobe Illustrator, which was used to print them out using an Epilog 60W laser.

Print the mold files in order using settings appropriate for cardboard (I used 70% power, 90% speed, 500Hz frequency on the epilog 60W laser).

Each part should have numbers showing how to assemble them. I suggest dry-fitting the parts together to make sure they are all present and printed correctly.

Now assemble the parts with a little glue between each layer.

I chose not to clamp my mold parts together, because I didn’t want to risk shifting the pieces, and they seemed to hold well on their own. Others clamped their molds, which worked as well — do whatever feels right. Clamping probably helps keep the mold a little more compact.

Before laying up the carbon, we have to do a little prep work to the mold.

Soften edges. Carbon fiber does not bend well over sharp corners, especially the cheaper plain-weave fiber that I used. On this design, since the sides are all rounded, there is only one problem spot – the joint between the body and the neck. This sharp corner would cause the carbon fiber to pull away or tent in ways that don’t look very nice.

To ease the transition I used Bondo auto body putty to make a smooth curve. This could also be fixed by adding some more bulk here to the original mold file.

Make a handle. I nailed some pieces of scrap wood to the bottom of the mold (top of the guitar). The wood gave me a firm handle for the mold that could be put into a vice. The vice elevated the mold and kept it rock steady while I worked. I also suggest wrapping the vise in plastic wrap to keep it clean.

Release the mold. To avoid the epoxy bonding with the cardboard, we need some type of barrier. There are commercial mold release products that chemically separate the mold from the epoxy, but that won’t work well with the cardboard molds. For this workshop we went with a low-tech solution — wrapping the entire mold in packing plastic — a thicker, tougher cousin of cling-wrap. There’s no “right” way to wrap the mold, but try to aim for long, smooth motions to avoid tangling and bunching of plastic. Make sure everything is covered.

If you use another mold material with a smooth surface, such as foam or plaster, then commercial mold-release agents such as TAP Plastics’ “PVA mold-release” are probably a better idea because they ensure the mold can be reused.

Unfortunately I don’t have detailed pictures of this step, because as you can see it’s quite messy. I had to rely on passers-by to help me take pictures. But it’s pretty self explanatory, and I’ll do my best to explain the process in writing.

Neatly arrange everything before starting. Once you mix the epoxy together, things will start to get very messy very fast. And every pot of epoxy has a hard time limit before it hardens. Mine had a 20–30 minute pot life. If you are familiar with cooking, think of a “mise en place” style workspace.

TIP: Protect the workspace by laying down some more plastic wrap or tarps over the floor, table, and vise

Cut the sheets of carbon fiber and S-Glass to size by laying them over the mold, cut them just a little longer and a little wider than the mold (I had about 1/2″ on each side). You don’t want too much material, because the extra weight will cause it to drape in funny ways.

Save one special layer of carbon and set it aside. The first few layers can be as messy as you please, but the final layer will be your show-layer. You want it to look as nice as possible, without any warping for fraying.

Once the materials and tools are ready, mix up the epoxy. Carefully follow the mixing instructions of whatever epoxy you are using. In my case, it said to mix 4 parts resin to 1 part hardener.

• I started mixing it by eye in a measuring cup: 8 ounces resin, then pouring in 2 more ounces hardener for 10 ounces of epoxy. This worked OK, but it’s not the best method. A better way is to use a scale to measure out be weight. I always weighed later mixtures.
• Epoxy likes it warm, so be aware of the work room temperature. This workshop was held during a cold-spell and it took several hours longer for the resin to dry than it should have. The epoxy instructions should include a minimum recommended temperature.

Lay the first layer of carbon over the mold, then load up your brush and start spreading epoxy around the surface. Once the epoxy hits the fiber, it will instantly start to hug the curves of the mold. The large curves are easy, but once you get into tight areas it can get tricky.

• Tight corners like around the neck will make the carbon fiber tent and pull away from the mold. If you are using plain weave fiber like me, you can’t win this fight. Instead, cut some relief slits down the middle of the area that is pulling away, then over-lap the two sides of the fiber. Don’t worry about how this looks, since this is only the first layer.
• You want all the carbon fiber to be thoroughly coated with epoxy, no dry spots or bubbles. It can be hard to tell with pure black carbon fiber, but just examine each section carefully and look for differences between wet and dry spots.

Put down a layer of S-Glass in exactly the same way. It’s easier to tell when fiberglass is fully epoxied, because it becomes transparent when wet. If the S-Glass also tents or pulls away in tight curves, cut relief slits just like with the carbon. If necessary, cut some extra scraps of S-Glass or carbon and bulk up the sharp corner to soften the curve.

Finally, lay another layer of carbon down. By now the sharp corners should be softened, so try to lay these lay these layers down without cutting relief slits.

When it’s time to lay your show-layer down, be as careful as you can to not pull or tug the carbon out of place. Everything you do now will show on the surface.

Layup tips:

• I mixed one batch of epoxy per layer of carbon fiber per S-Glass. This ensured I was always working with fresh, easy flowing epoxy and it gave me time to relax and get the details right.
• Use your brush to apply the epoxy, but don’t be shy about using your (gloved) hands. Tug, push, smooth and reshape the fiber by hand, especially on the under-layers.
• Try to get the final layer of epoxy looking nice, but don’t over-think it. The final step in my whole guitar building process was to go back and sand the body smooth before applying one more gloss-coat of epoxy.

Once you are happy with the result (or the epoxy has started to set) step away from the mold and just let it dry for as long as it takes. Don’t touch the drying epoxy. Any fingerprints or dents will become part of the final product. It’s possible to fix later, but much harder than getting it right the first time.

After drying, the epoxy will be firm, but the surface may still be sticky. My epoxy took much longer to dry than it should have, partly because of my imprecise mixing method, but also because the workshop was colder than it should have been. In the end I used TechShop’s powder coating oven set on low (under 120ºF) to make sure it was fully cured. If you are at home, just keep the room warm.

There will be a lot of excess carbon and epoxy fringe handing off the bottom of the mold. Be very careful when removing the fringe since the little spikes can be sharp as knives and make some pretty nasty splinters when broken. Use gloves, and definitely use a dust-mask, or better yet a respirator.

Cut away all the extra material to just above the cardboard mold. Some people used an angle grinder or Dremel tool, but I preferred to take it slow and use a hacksaw. When hacksawing I tried to saw off large sections at once, giving the saw a lot of material to bit into, instead of sawing each small part, which caused the saw to flex.

After the extra material is cut away, you can either tear up the mold from the inside out, or try to pop it out in one whole piece. To pop the mold out, it really helps to have a second set of hands. One person can hold the guitar body steady while the other tugs on the mold. Carbon fiber is very tough, so don’t worry about pulling on it, but don’t go crazy either. It is possible to snap thinner parts right off.

Once free from the mold, examine your handiwork. The hardest part is now over, the rest is just details.

The role of the guitar body is to be solid and steady, without too much flexing. Any energy spent bending the guitar body is energy taken away from the sound of the instrument. I went around the instrument and looked for any areas that were flexing too much.

Most of my body was very solid, but the edges flexed just a little bit, especially at the “hips” around the center of the guitar. Also, some parts of the neck were a little less stiff than I wanted. The neck in particular must be very strong, to withstand more than 100lbs of pull from all the strings. A traditional guitar has a “truss rod” — a strong threaded rod that can adjust the neck stiffness. It’s probably possible to make an adjustable truss rod for this style of guitar, but we chose to just make a very stiff neck, reinforced with Coremat.

Coremat is a super-absorbant foam that sucks up lots of epoxy, making it very hard and stiff once it dries. I put a layer of Coremat all along the edges of the guitar and down the center from the neck. The Coremat by itself does not stick well to the carbon fiber, so I put a strip of S-Glass under the Coremat to help secure it. This application of the Coremat also has a secondary benefit, which I’ll discuss in the next step.

It’s not pretty, but nobody will see these parts anyway.

The next step is to prepare the guitar for the wooden components.

The Coremat: While the Coremat from was dying, I put the guitar upside-down on a plastic mat and let the epoxy drip down and form a “lip” around the entire edge of the guitar. This lip will make a nice surface for mounting wooden components later. I also weighted the neck and body down so that they would dry flush to the table together.

Flattening the body: When laid flat on a table, the neck and body of the guitar should be flush. If either the neck is higher or lower than the rest of the body, it will cause problems with the action (pathway) of the strings. In my case, my body was very flat with just a few high spots. To flatten it I put some 80-grit sandpaper on a flat 2×4 section that was longer than the guitar body was wide, and then sanded the edges until they were flat.

Make space for the soundboard: Finally, with the overall body flattened, I cut a space for the soundboard to fit underneath the fretboard.

1. Measure the thickness of the soundboard precisely (mine was 2.6mm thick).
2. Using painters tape (or other contrasting material) mark off a line down from the top edge of the guitar body. The goal is to make a lower lip where the soundboard can sit, allowing the strings to run straight off the fretboard to the bridge.
3. Use a Dremel or angle grinder to cut to the line of the painters tape. I wasn’t confident with this part, so I got the instructor to cut my lip for me. Instead of measuring, you can also put your soundboard next to the body (turned upside down) and draw a line directly onto the body using the thickness of the board as a reference.
4. The goal is a nice pocket where the soundboard can sit.

While cutting the body, also take time to look at the size of the neck. In my case, I haven’t played a guitar in years — I had no sense of how thick the neck should be for my playing and hand size. I left the neck thicker than it needed to be. My fingers are long enough to reach around it, but I have a feeling it will be annoying when playing for an extended time.

If I could do it over again, I would cut both the neck and body down thinner. I would go to a music store and get a feel of some commercial guitars.

All of my wooden components are basically glued directly to the body with 5 minute epoxy. First the soundboard is attached, then the headstock, and finally the fretboard and bridge.

There are many important details about installation, and I’m not confident in my ability to explain them yet. So I strongly suggest finding a professional guide to guitar construction. I used Build Your Own Acoustic Guitar by Jonathan Kinkead, which I found at my local library.

The Soundboard

• Spread 5-minute epoxy around the perimeter of the guitar, using the lip created by the Coremat reinforcement.
• Press the soundboard flat against the body, making sure it is centered.
• Make sure the bracing on the back of the soundboard is not touching the sides of the body.
• Since the body doesn’t have a flat back to clamp against, I turned the guitar upside down and placed a heavy lead weight on top.

The Headstock

• I cut the headstock portion of the carbon fiber to fit my wooden headstock using a Dremel cutoff tool.
• I spread 5-minute epoxy onto the Coremat inside the headstock and neck.
• I fit about a 2″ piece of the headstock wood inside the neck.
• I test fit the fretboard, to see if the end of the headstock was sticking out above the neck.
• I sanded down the small piece sticking out with 80-grit sandpaper until the fretboard rested flat.

The Fretboard

• I cut the fretboard to fit the neck and circle around my sound hole.
• I spread 5-minute epoxy over the Coremat inside the neck then installed the fretboard.
• I firmly clamped the fretboard, with F-clamps placed every couple inches. I also used a long-reaching C-clamp inside the sound hole to clamp the bottom of the fretboard.

The Bridge: The bridge must be centered and it must be just the right distance from the nut at the top of the fretboard. The model my guitar is based on calls for a 63cm distance from bridge to nut. For custom designed guitars, there is also a formula to determine the proper distance, and how to combine that with a proper fretboard layout. This is the type of thing where a professional guide is essential.

NOTE: I weighed the bridge down on the soundboard and glued it with 5-minute epoxy. This worked for about a month, but then the bridge started to separate from the force of the strings. Luckily for me it was a clean break and nothing else was damaged. When I repair this I’ll use wood glue and a proper clamp.

Before finishing the body, I went back to fill in some small (and not so small) gaps between the guitar body and the other components.

Small gaps / holes: Mix up a little epoxy (5-minute or slow-cure, as you prefer). Add some black pigment to match the carbon fiber. Use a toothpick or other fine tool to poke the epoxy into the holes.

Larger gaps / holes: On my guitar, I only had one larger gap to fill — a space between the top of my headstock and the bottom of my fretboard. I used the same method as above, but I also stuffed some loose carbon fibers in with my black epoxy to give the patch some body and hold the epoxy in place. I later sanded this patch flat, flush with the neck.

What is perfect? You can spend a lot of time filling every little hole and sanding everything flat, but at some point you have to call it a day or think of other strategies.

In my case, I plan to go back later and make some kind of trim (like purfling on a traditional guitar) that covers the edges of the body and soundboard. So I only bothered filled the largest holes, and areas that might not be covered by the trim like the above headstock-fretboard joint.

At this point, you should have a nearly functional guitar. Make the frets, put some strings on and it’ll be ready to play. Trouble is, all the cutting, sanding and scraping can make the body look awful. And residual stickiness from the epoxy will attract all kinds of dust, lint, trash and stray carbon fibers. One person in the workshop called his guitar “Urban Blight” at this stage.

But, with some sanding and polishing, you can turn that all right around. Here’s what I did to make my guitar look pretty again. You may notice some blemishes on the finished product — I think something touched the final coat of epoxy while it was still wet. I could have added yet another coat to fix these spots — but at this point I just wanted to finish, and it wasn’t worth another day of drying. I can always fix it layer if I want.

Protect wood components: My method of finishing uses wet sanding, which can damage unfinished wood components. I suggest finish-coating the wood top and protecting the other components. I kept my bridge and fretboard covered with painters tape for this step.

Level sanding: I had some big ridges of un-evenly dried epoxy on the bottom of my guitar, as well as many small drips and runs. I sanded these down with a sanding block and 80 grit sandpaper.

Use coarse sandpaper sparingly — epoxy isn’t easy to break, but it can easily be scratched away. I only used this for the highest of high spots, and frankly 120 or 180 grit might be a safer choice. Once you get everything mostly even, move up to a higher grit and start wet sanding.

Wet sanding: Wet sanding is exactly what it sounds like, you use waterproof sandpaper (black or grey, as opposed to brown or red) and frequently rinse the area being sanded with water.

Wet sanding is good for hard clear-coats like epoxy because it keeps dust from settling into the surface and makes the sandpaper more efficient.

• Using waterproof sandpaper, I started with 180 grit, then moved up through 220-, 320-, and 400-grit papers.
• The surface always looks PERFECT when wet (very deceptive). So between grits I wiped away the muddy dust with a damp rag and let it dry a minute.
• Once dry, I wiped away any remaining dust with a tack cloth and looked at the surface. I was looking for the scratch pattern, a uniform grey with evenly spaced and sized scratch marks.
  • Larger, deeper scratches remaining after a thorough sanding, they are left over from a lower grit of sandpaper. You may have to drop down to a lower grit to remove these before continuing.
  • Shiny spots are areas that have not been sanded. You want to sand everything evenly before moving to higher grits.

Hopefully the surface is starting to look good at this point, but there are probably some flaws. In my case, I had millions of tiny air bubbles trapped in my. This made the finish beautiful in some places, but very cloudy in others. To fix this I thoroughly cleaned the surface of the guitar, then added one final gloss-coat of epoxy. I carefully measured out the epoxy with a kitchen scale, then brushed it on with a foam brush, and I let it cure one whole day in a warm room.

In addition to epoxy, you can also use other finishes I saw one person use polyurethane, and another use black tinted epoxy for a matte-black finish. But I wanted the carbon fiber weave to show through on mine, so I just used the same marine epoxy I had used to form the body.

Continue wet sanding: After the new layer of epoxy dried, I started sanding again with 400-grit paper, then 600 and 1000 grit. At this point, it had a semi-gloss appearance, which is all I needed. For a glossier look, keep using higher grit papers, then polish.

Polishing: After sanding I used some auto-body rubbing compound (Turtle Wax). I rubbed it on thoroughly with a buffing pad, then gave the guitar a wipe-down with a damp rag.

The last thing I did was rub on a little paste wax, let it dry, then hand-buffed the body to a semi-gloss shine. I don’t even know if this is appropriate, but it’s what I’ve done with lacquer finishes on wood, and I like the look, feel and smell of the wax, so I’m glad I did it.

This project pushed me to learn a lot of new skills and try experiments I never would have attempted otherwise. The fretboard is inlaid two ways: with CNC-cut maple and with laser-cut lettering filled with colored epoxy; the headstock is also laser-inlaid and the bridge is CNC-cut too. Finally, I laser-etched the Western red cedar soundboard with my own artwork, then hand-tinted it.

Considering this was my first time working with carbon fiber — or making a musical instrument — I’m very happy with how it all turned out.