599 undertray repair

Thought I would share my attempt at repairing the stress cracks on the giant underpanel tray- the 430 forum seemed to have a lot of users who identified tray issues with DYI repairs as well.

I haven’t finished all areas on my tray yet, but I have some test spots to evaluate and plan to touch up refinishing efforts if I like the results.

I went with a carbon fiber patch approach and sanded before using a wet lay up with pre-ply to finish.

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I don't have too much experience in this area either, have played around with making fiberglass brake ducts on a rx7 about a decade earlier.

While the tray looks to be some plastic/fiber composite, I decided to try a carbon patch because:

1. I hate working with fiberglass - itchy fibers
2. Tray's plenty heavy and I was looking for better strength-to-weight ratio in the stress areas which I was optimistic could mean applying less layers, especially in the preventive areas where I see hairline cracks materializing to arrest propagation growth

Steps:

• Degreased / cleaned undertray
• Identified and marked areas for repair (used blue tape, couldnt find a marker handy)
• Sand repair areas with 400 / 800 grit to create a mechanical surface for adhesion, then cleaned off the dust
• Couple areas with delamination or fully cracked, I used a dremel and cleaned up the edge lines to encourage better bonding with the repair patch
• Used patch kit, cut-to-shape the carbon fabric for the repair locations, I planned for two layers in most of the repair spots that just needed reinforcement on the internal side of the tray, mixed a small batch of epoxy + hardener 1+1, then just applied a thin layer of epoxy/hardener mixture, pressed down the carbon fabric, added another layer of epoxy, added carbon fabric, then added peel ply on top and pressed down with a scraper to move out excess epoxy
• I have a couple areas that have bigger damage that required using a dremel wheel and cutting to shape a new piece to bond on. I dremeled out the shape on a carbon panel, and repair patch bonded the sheet using some clamps
• Tricky areas with 90 degree bends I generally added some clamps during curing to hold shape
• Let cure 12 hours
• Resanded surface area to remove excess epoxy and create a surface for clear coat bonding
• Got lazy and reinstalled as-is, but I plan to use a matte clear for better finishing the next time I inspect the tray

Preliminary results:

• Good bonding adhesion if you surface prep appropriately with sanding. My first patch attempt I did not do that and I removed and redid
• Lightweight repair
• two-layers and a one-sided repair seem sufficient in strength, but out of prudence I plan to go back and add another layer or two on the opposing side for additional strength for the major repair areas that cracked completely through.

Supplies I used:

• https://www.amazon.com/Twill-Carbon-Fiber-Epoxy-Resin/dp/B0785NLJHD/ref=sr_1_3?crid=2SWB23V7J5ZVS&keywords=carbon+fiber+patch&qid=1692380457&sprefix=carbon+fiber+patch%2Caps%2C180&sr=8-3
• https://www.amazon.com/Fibre-Glast-Econostitch-Package-Visible/dp/B07DP4L3V3/ref=sr_1_1_sspa?crid=132FBTY1YNI9N&keywords=peel+ply&qid=1692381066&sprefix=peel+pl%2Caps%2C212&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1
• https://www.amazon.com/TOLESA-Powerful-4-Piece-Clamping-Securing/dp/B087JH9YGN/ref=sxin_16_pa_sp_search_thematic_sspa?content-id=amzn1.sym.10e6d77b-0012-4a6b-b8f0-1618f27938ee%3Aamzn1.sym.10e6d77b-0012-4a6b-b8f0-1618f27938ee&cv_ct_cx=clamps+for+woodworking&keywords=clamps+for+woodworking&pd_rd_i=B087JH9YGN&pd_rd_r=e9b4e415-3140-4998-bda8-8eea60eef387&pd_rd_w=M64iH&pd_rd_wg=K3urN&pf_rd_p=10e6d77b-0012-4a6b-b8f0-1618f27938ee&pf_rd_r=70CTNGSFTZ9CY55M9ECA&qid=1692381491&sbo=RZvfv%2F%2FHxDF%2BO5021pAnSA%3D%3D&sprefix=clamp%2Caps%2C223&sr=1-3-2b34d040-5c83-4b7f-ba01-15975dfb8828-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9zZWFyY2hfdGhlbWF0aWM&psc=1

 

the peel ply helps draw the excess epoxy out to the top of the cure, which helps provide a better surface to sand and paint/clear coat. (So I’ve read.). That does seem to be the case and I was able to sand the surface easily for something smooth. it also has the benefit of a release agent on it, so when I use a clamp to press the carbon through the 90 degree bends, the clamp won’t cure stuck to the epoxy, it can release easily with the peel ply acting as a barrier.

 

Happy to share and looking forward to learn from others.
Going to tackle sticky buttons and shrinking leather dash in the next few months and plan to document too…

 

Following up since I have since taken my undertray off again to inspect how the repair has held up. Pleased to find it all doing well, so I picked up the project from where I left off and tackled repairing it from the other side for double reinforcement and also for the more complicated tab repairs. I used a dry carbon plate and cut out pieces of the chunks missing from my undertray to fab on using a dremel and diamond carbide cutting tools.

then tons of clamps, epoxy, and another layer of carbon to connect it all together.

and I used a matte clear to finish off.

“weekend arts and crafts by the beach” means something different to me than most folks

 

Lowering definitely raises risks. From what I can tell on my car, it’s predominately driven from rough handling during maintenance and just plain brittling from the fibers aging. I plan to lower to HGTE height in the next month. Just picked up springs from a forum member as well as putting on the HGTE exhaust.

 

So excited to check out the finished product after the cure today- managed to rebuild the two clip mounts successfully!
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