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As follow up on my thread about cracking resin on the carbon fibre door sills, (https://rennlist.com/forums/996-foru...air-ideas.html), recently the cracking appeared to be spreading and the resin coating lifting.



The following outlines an 80% fix (in my estimation) using fibreglass resin available at most auto parts stores (3M Bondo Fiberglass Resin). I have never worked with resin before and would rate this relatively easy to do, but I learned a bunch doing it and will share herein.

Total cost ~ $20
Total time – 2.5 hours (+2 hours drying time for the resin)

Tools

• Painter’s tape
• Sharp utility knife
• Picks
• Sand paper grades (220 to 2000)
• Power drill and polishing wheel (or equivalent)

Preparation

First, I taped off the working area to minimize the potential for scratches.

I removed all of the old cracking and lifting resin using a sharp knife and a couple picks to gently pry off the existing clears coat. Cut straight lines around the working area to limit the amount that breaks off. The resin coating has a tendency to lift, so delineating a working area by cutting into the existing good resin is critical. You need to cut all the way to the carbon fibre base. I should have done a better job of doing this everywhere. I would also suggest bevelling the edges to get a better adhesion and transition between the old and the new surfaces.

I sanded the area around the area to be repaired with 220 grit to create better adhesion.

I then cleaned the area with compressed air and wiped it with acetone.



Repair

Mix up the resin and apply liberally to the area with a paint brush. You need to fill a better part of 3/32” (2.5 mm). You will also need a rag to continuously remove the overflow of resin to keep it off the painted part of the sill.

I had a great amount of difficulty getting the areas to fill near the curve of the sill (gravity was working well today). It took two applications of resin to achieve a reasonable result (but not great). On the second coat, I deliberately mixed more hardener and waited about 5 mins for it to thicken before I applied it. I also used a plastic putty knife to apply it, to get a better adhesion in the curve. This produced a reasonable result, but resulted in a rougher surface than I would have liked.

Note: the tape is critical in keeping the runoff resin from sticking to the painted part of the sill. However, do not wait to long to remove the tape as the resin hardens it becomes difficult. I ended up waiting about 1 hour and had trouble. I used my knife to cut the resin at the edge of the sill, but it was a delicate and difficult operation. Next time I would test it after 30 mins.

Let dry for 2 hours.



Finishing

Nothing hard here, just lots of wet sanding.

I started with 220 grit to smooth the roughness and create a smooth transition between the old coating and the new resin (I actually used my Ryobi orbital sander to do this). Then by hand did the following (probably overkill):

320 grit
600 grit
800 grit
1000 grit
1500 grit
2000 grit

I then polished with my electric drill using

1. Meguiar's #2 Fine Cut Polish– 2 passes
2. Meguiar’s Plastic-X Polish – 2 passes

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The outcome is acceptable in my opinion. There is a bit of bubbling under the old resin which shows up in the pictures and the area under the rubber weather stripping is a bit dull in sheen and less smooth, but otherwise I feel better off that when I started. The flash actually makes the repair looks poorer than it is to the naked eye. Next I will repair the two cracks in front of the area that I just repaired.

Divot

Good job.

(but you left out the most important ingredient - Stella.)