Wheaties started the topic Composite under body cover inspection/modification in the forum Model 3 2 years, 9 months ago
Underbody composite material fails because bolts can rattle loose and cover absorbs water and can act as air/water scoop. Check bolts for tightness and apply thread lock to bolts and consider applying NeverWet to prevent water absorption.
There is a composite fabric under body cover that appears to have multiple issues that can lead to failure. Take a moment and check the existence and tightness of the bolts holding the composite cover under your vehicle. I was not aware this was even an issue until my cover failed.
My cover failed in only 4 months of ownership and under 4,000 miles. Researching other forums/threads following the failure, I have learned that the bolts holding the cover are found to be loose and susceptible to falling out. The material also can absorb water which would allow expansion of the material and sagging. The combination of both can lead to the cover becoming an air and water scoop moments before it tears itself apart on the road.
Upon inspection following the failure, I found one bolt missing and the majority of the remaining bolts were removed with my fingers. I removed the remaining cover and promptly headed to the service center. The personnel at the service center replaced the cover but they did not have a bolt to replace the missing bolt. Once returning home, I removed the cover and it was already wet and absorbing water still on pavement following the previous days rain.
It was at this point, I decided to add some water protection to the cover and thread lock to help secure the bolts from rattling loose in the future.
I still had a quite a bit of Never Wet in my garage. NeverWet is a hydrophobic spray that creates a moisture repelling barrier. You can find this at Home Depot, Amazon and many other big box stores.
I still needed the final bolt and thread locking gel. I learned thru other threads on this issue, that missing bolts could be sourced at Home Depot, so off I went to get the missing bolt and thread lock.
Having all the materials, I allowed the cover to dry overnight. The NeverWet is a two step process. Step 1 apply the base coat and allow to dry min 30 minutes. Step 2 apply the top coat and allow to dry. Several hours later, I applied the thread locking gel to each bolt as I installed the cover and that is all, easy peasy.
NeverWet is a nice to have but I believe the thread locker is important as the composite does not allow adequate tightening as the material is expandable/compressible. I hope by doing both, I will not have to deal with this issue again in the future.
This is not a critical part, but when you are at speed on the highway when it self destructs as it did for me, you believe just as I did that something very important just failed. The remaining portions of the cover thumped against the bottom of the car creating a loud thump as well as feeling the thump thru both the pedals and steering wheel.
I hope this alerts you to the issue and allows you the opportunity to catch part before it fails.