Instructables

Step 13: Sanding, finishing & polishing

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 looking. 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 in the final photo - 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 - this can damage un-finished 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 this corse 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 tiny tiny air bubbles trapped in my finish - millions of them. 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.
 
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DanW1311 months ago
Actually not too bad for your 1st attempt, I bet after you sleep on all the steps involved you could refine yoru work and make a guitar just as good as some of those "Pro's" Look's Good !!
workislove (author)  DanW1311 months ago
Thanks. I mostly do woodwork, and this was my first time trying composites. I'll definitely be returning to this type of thing in the future, maybe even another instrument - but taking it slow for now
vnams1 year ago
What kind of carbon fiber cloth did you use? You mentioned that you used some inexpensive cloth - would you advise someone to use a different kind?
workislove (author)  vnams1 year ago
This is the specific carbon fiber I used, straight weave. It worked fine, but it was a bit hard going around tight corners. This twill-weave carbon fiber is supposed to be better than straight weave for wrapping 3D objects, but I can't say anything from personal experience.

This is the other material I used, called S-glass. It's a type of fiberglass. The only reason I used it was to add bulk and stiffness to the shell inexpensively. Fiberglass is pretty strong, but not as strong as carbon fiber, and it also looks strange with epoxy - the moment it soaks up the epoxy it becomes translucent, and you can't see the pattern anymore.

I did this as part of a workshop, and it was my introduction to composite materials - so I can't say a lot about the different options.
thassaj1 year ago
Impressive! Would be nice to hear a few chords played on it so we can hear how it resonates.
gn0stik1 year ago
No pic of the completed guitar? Oh, come on!
workislove (author)  gn0stik1 year ago
Haha, i did all the parts as separate instructables. Didnt think to add finished pics here. But thats a good idea, ill add them later.
alphydan1 year ago
Beautiful work! Congratulations