Why Is the Back of My Carbon Fibre Part Painted Black?

If the back of your carbon fibre part is painted a flat, solid, opaque black, the most likely reason is that there is fibreglass underneath the carbon and the paint is there to hide it. A part that is carbon on every layer has no reason to be painted on the back, because the dark grey weave is already visible through the resin. That one look at the back face is the fastest check you can run on a part, and most people never think to do it.

I get asked this a lot, usually by someone who has already bought the part and turned it over in their hands afterwards. So here is the honest version of what is going on.

What a painted black back face is usually hiding

Almost every budget and mid-tier wet carbon part sold today is built the same way. The first ply laid against the mould is carbon fibre, and that is the weave you see on the finished face. The plies behind it are fibreglass, used as a cheaper structural core. The whole stack is vacuum bagged and infused with resin in one shot.

The problem for the seller is that fibreglass cures to a pale, translucent, off-white or yellowish colour. On the back of a black interior trim piece it is completely obvious. A coat of flat black paint makes it disappear.

To be fair about it, a painted back is not automatically a scam. Some parts get a back coat for UV protection where the underside is genuinely on show, like a bonnet vent or an open engine bay panel. But when the back is flat opaque black, with no weave visible anywhere, on a part nobody is ever going to see the back of, that is a question worth putting to the seller before you pay.

Why fibreglass backing exists at all

It exists because it is cheaper, and it works. Both types of wet carbon use the identical process, called VARTM, or vacuum assisted resin transfer moulding. Dry fabric is cut and stacked in the mould, a vacuum bag is sealed over the top, the air is drawn out, and liquid resin is pulled through the whole layup under vacuum pressure. It cures at room temperature or with mild heat, so there is no autoclave and no cold storage involved.

The only thing that changes between a fibreglass-backed part and a full carbon one is which fabrics go into the mould before the bag seals. Same vacuum, same resin, same cure. That is worth understanding, because plenty of well made parts are built with a fibreglass core and there is nothing sloppy or amateur about the method. If you want the wider material picture, I have written before on how carbon fibre, fibreglass and ABS compare as materials.

The issue is not the method. The issue is paying full carbon money for a part that is mostly glass by volume, and having no way to tell because the evidence is under a coat of paint.

How to check a part yourself, in order

This is the order I go in when a part lands on the bench.

• The back face first. Flat, solid, opaque black with no weave anywhere is the single most common tell on the market today. Full carbon shows dark grey weave through the resin on the back, because there is nothing to hide.
• Cut edges and mounting holes. Any trimmed edge or drilled hole gives you a cross section. Carbon plies read dark grey to black all the way through the section. Fibreglass plies read pale, sometimes almost translucent. On interior trim the clip locations and mounting holes are the easiest places to see this.
• Weight in the hand. Fibreglass is denser per unit volume than carbon, so a glass-backed part feels heavier than a like-for-like full carbon panel of the same size.
• The tap test. A glass-backed part tends to sound duller and deader than full carbon. This one is the least reliable of the four, use it to confirm rather than to decide.
• Just ask. Send the seller a message asking whether the structural plies are carbon or fibreglass. Anyone selling full carbon will answer that in one line. A vague answer is an answer.

Worth saying that this is a different question to whether the part is carbon at all. A hydro-dipped or vinyl-wrapped plastic part is a separate problem with separate tells, and I have covered how to spot fake carbon fibre before you buy in its own post.

Where to look on a Ford Falcon FG

FG interior trim is the easiest place to run this check, because it comes out. A shifter surround or a set of door pull cups can be unclipped, turned over, and inspected in a couple of minutes. Look at the back face, then look at the cross section around the mounting holes and the clip locations where the material has been cut.

Engine bay parts are even easier, because the underside is sitting right there once the bonnet is up. A coil cover or a radiator cover gets pulled and flipped without tools on most cars. If you are weighing up whether that upgrade is worth doing in the first place, I went through whether a carbon coil cover is worth it on an FG separately.

If you are past the research stage and want to see actual parts, the Ford Falcon FG carbon fibre range is here, and our full carbon fibre catalogue covers the other models.

Does the fibreglass backing actually matter?

Honest answer, it depends on the part. On a cosmetic interior trim piece that carries no load, the practical difference in daily use is small. Where it does matter is what you paid, how the part behaves under heat and UV in an engine bay or on an exterior panel, and what it is worth if you sell the car on.

There is also a simpler point. A maker who leaves the back bare is a maker who is happy for you to look at it. That tells you something on its own.

What sits in the RB Innovations range

The current range is prepreg dry carbon. Prepreg fabric arrives with the resin already in the weave at a set ratio, it is kept in cold storage until it is cut, and it cures in an autoclave under both heat and pressure, typically 120 to 180 degrees Celsius and 6 bar or more. That gets fibre content up around 60 to 70 percent by volume with very low void content, against roughly 45 to 55 percent for a VARTM part. That is why prepreg costs more at retail, and it is the whole reason for the process.

If a wet carbon part is ever added to the range, it will be full carbon on every ply, surface, structural and backing, infused with the same VARTM process. Not carbon over glass. And no painted back face, because there would be nothing to cover up.

FAQ

Is a painted black back always a sign of fibreglass?

No, but it is the most common reason by a long way. Some parts get a back coat for UV protection where the underside is visible in use. The test is context. If the back of the part will never be seen and it is still painted flat opaque black with no weave showing, ask the seller what is under it.

Can I sand or strip the paint off the back to check?

You can, but I would not. You risk heat or solvent damage carrying through to the clear coat on the show face, and you will devalue the part either way. The cut edges and mounting holes give you the same information without touching the finish.

Is carbon over fibreglass actually weaker than full carbon?

Per equivalent thickness, yes, carbon has better mechanical properties than fibreglass. Whether that difference is meaningful depends entirely on the part. A trim piece that carries no load will be fine. A structural or load-bearing panel is where the material stack starts to matter properly.

Does forged carbon get a fibreglass backing too?

It can, depending on the maker. Forged carbon is a purpose-built prepreg fabric made of short irregular carbon tiles pre-impregnated with resin and set in a random orientation. What sits behind that surface is a separate decision. Run the same back-face and cross-section checks on a forged part that you would on a woven one.

How do I check a part before I buy it online?

Ask for a photo of the back face and a close-up of a mounting hole or cut edge. Both are quick for a seller to take, and both answer the question. If a seller will not send a photo of the back of a part, treat that as the answer.

Written by Riley Baginski, founder of RB Innovations.