I have a carbon fiber composite component that is often and repeatedly subjected to stress. When it was brand new, everything looked normal. After a week of use, there are what seem to be bubbles developing under the clear coat.

1) Why would there be bubbles developing under the clear coat? Wouldn't the manufacturing process of heating and vacuuming bagging have eliminated most of the air within the composite?

2) Does the presence of these bubbles indicate a manufacturing defect?

My suspicions are that these bubbles are merely a cosmetic defect as I can see them, which indicates that they are not in between the layers of unidirection fiber which actually gives the structure its strength. On the other hand, the presence of the bubble does make me wonder - are there possibly large voids within the structure? From what I understand voids weaken carbon fiber composites, correct?

  • 2
    $\begingroup$ Can you post a picture? $\endgroup$ Commented May 21, 2015 at 11:50
  • $\begingroup$ What sort of component are we talking about? Tube, panel, etc.? And in what configuration/application - so we have an idea of the magnitudes of stress/deformation. $\endgroup$
    – Air
    Commented May 21, 2015 at 15:05
  • $\begingroup$ @Air - it's a tube. $\endgroup$
    – Dissenter
    Commented May 22, 2015 at 18:38
  • $\begingroup$ I'm going to go out on a limb and guess, based on some of your recent network activity, that the component in question may be part of a bicycle frame. This is potentially important information for us to have; it can tell us a lot, not only about the stresses on the component but also about likely weathering and manufacturing processes, for example. Please don't be stingy with the details, it makes it that much harder for us to give your question good tags and great answers. $\endgroup$
    – Air
    Commented May 22, 2015 at 19:07

1 Answer 1



Since you mentioned that the "bubbles" appear to be under the clear coat, it is likely caused by a failure of the bond between the clear coat and the material underneath. This is especially likely since you said that the part is subjected to repeated stressing. The clear coat, carbon fiber and epoxy are different materials and have different physical properties. The stress causes each material to act differently.

This doesn't mean that there were bubbles to begin with. The clear coat is very good at transmitting light, so discontinuities will show up readily. You would have been able to see them initially if they were there.

Manufacturing defect

It is really hard to tell from your description if this is a manufacturing defect or not. It could be, but it also could be caused by using the part in a way that it wasn't designed for.


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