Patching holes in a carbon fiber body tube?

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Sep 27, 2022
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Hello all!
I have a 3" diameter carbon fiber body tube that my rocketry team inherited from a previous, now defunct, rocketry group at my university. My team and I are looking to make some repairs/modifications to the tube to transform it into a flight-worthy rocket. One of the issues with the body tube in its current state is that there are 20 small holes that were drilled into it in seemingly random places. I'd like to get these holes patched so that when we paint the exterior later, you would never know that the holes were even there. This is less of a functional problem and more of an aesthetic issue since there aren't enough holes to significantly compromise the integrity of the body tube. Any advice on what kind of material/method we could use to patch these holes so that the patches are flush with the outer diameter of the body tube? My initial idea was to put some wax paper on the outside of a coupler, slide the coupler inside the body tube, use epoxy to fill the holes, then put tape over the holes as the epoxy dries so they shape correctly with the curvature of the tube. Thoughts?IMG_2053.jpg
Yeah, that'll work. Consider using mylar instead of wax paper. Maybe saran wrap, if you can get it to lay flat enough on the coupler so it slides in easily.

I get 36" wide 1 mil and 5 mil mylar from my local TAP Plastics. Nice stuff!

Packing tape is good stuff, too, if you can use the adhesive to your advantage. I use it to close the seam of the 1 mil mylar wrap. Might be too tough for Saran wrap?
Your idea is perfect. But find some scrap CF, chop it into fine strand and mix that in with your epoxy. After a light sanding to blend the outside you will never even see that holes.
That's a lot of holes and they are in proximity of each other. That has structural effects.

Do you need to be able to slide anything inside the tube in the region of the holes afterwards?

Clean the inside and outside of the tube well, with denatured alcohol. Don't touch the regions with hands afterwards.

Slightly chamfer the inside and outside of the holes, via sanding.

Sand the inside of the tube in the patch areas.

Clean it again.

Use mylar sheet on the outside (waxed), conformed to the tube shape.

Fill from the inside with structural thickened (thixotropic) epoxy.

If you can, put a composite fabric patch on the inside to stabilize the plugs. Where there are multiple holes in an area, make it one patch over all those holes.

Use enough fabric thickness/weight to actually stabilize those patches against moderate impact. That may require a patch of decent percentage thickness of the original tube.

Don't try to fix them all at the same time. Gravity may not be your friend!

There are other methods of course.

Your idea is perfect. But find some scrap CF, chop it into fine strand and mix that in with your epoxy. After a light sanding to blend the outside, you will never even see those holes.
Adding to what David Schwantz and Gerald/G T offer, consider:
Wildman sells smallish jars of chopped carbon fiber (to blend into slow-cure epoxy) that add considerable reinforcement/strength, and would also blend in well, as you repair those holes once they're properly peri-sanded and beveled (both internal and external of airframe).
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Just put a piece of painter tape over the inside of a hole. The adhesive on the tape will release the cured epoxy well enough when you later pull the tape out. Easier that getting a coupler with layer of release film on it positioned correctly, and less risk of a bleed-around bonding the coupler in place.