Ultra low density SMC SNAFU at the core of all Corvette C7 Stingray paint quality and cracking panels problems-a dirty secret General Motors hides from the buyers

The science of composites is not just a science, it is also an art.  When developing new materials, sometimes there is a meaningful progress and sometimes there is a failure.  When the failure is discovered during the development stage, it is not a big deal but... if it is discovered after production run starts, it becomes a true nightmare.

+General Motors already demonstrated what it does as a corporation after problems in production cars are discovered-absolutely NOTHING, just like with the faulty ignition switches.

Thus, it is no surprise that the same strategy was adopted by +Tadge Juechter and company, regarding the marketing bait he loves to wave around, the ultra low density SMC body panels the newest Corvette uses in a FAILED attempt to reduce its weight.

SMC or sheet molded composites have been around Corvette since 1972, with not much change occurring until about 1999 when GM started pursuing the venue of low density SMC materials.

The first Corvette that used low density SMC was the C5 hard top coupe and Z06.  At that time, the technology did not allow for using low density SMC for outside panels, thus, only the inner part of the roof structure of that car utilized the low density SMC.

Due to the problems with the lack of paintable surface on low density SMC panels, they were not used on the sixth generation on any outer panels (this is the reason why so many previous generation current Stingray owners are puzzled by better paint quality prevailing on the previous generation of the Corvette.)

However, things have changed big time with the latest Corvette.  There is no more high density SMC used for the outer panels, all outer and non carbon fiber parts are of not just low but ultra low density SMC.

What is exactly ultra low density SMC?  As a matter of fact, it is not all that different from low density SMC, just even lighter, with more filler and less matrix and resin.

Ironically enough, the filler General Motors uses, the glass microspheres, is the same filler the author of this blog uses for his composite adventures. 

What is exactly the purpose of glass microspheres filler?  MASS REDUCTION due to lesser density is the reason.  The microspheres are hollow, the resin bonds the microsphere outer walls and air is trapped in the material.

The benefits of microspheres are numerous: of course there is the mass reduction but there is also less shrinkage due to lesser resin content and microspheres retaining their shape while remaining hollow.

This lack of shrinkage, incidentally, is the reason why Juechter mouthpiece has been bragging about body panel gap reduction.  When panels shrink less, there is less leeway to account for thermal shrinkage and expansion.

However, since nothing is perfect, there are also disadvantages to using this glass microsphere rich material: there is loss of strength (not just resin but also glass fiber matrix amount was reduced).  The loss of strength of the new material is very obvious with the cracked fender panels that Bowling Green employees are busy patching up using epoxy.

The other drawback is the lack of suitable paintable surface.  The problem with microspheres is that when sanded, the surface becomes porous, containing many micro pinholes.  Even though these pin holes are not visible to naked eye, they come out when a panel is painted.  The other problem with this new SMC material is the use of polyester resin instead of the vinyl ester resin used in the past.  The vinyl ester resin is costly but it gives considerably higher quality and more consistent final product.  This part was not obvious with C5 or C6 cars because at that time, calcium carbonate was being used as a filler.

Calcium carbonate as a filler allows for high quality A class body panels, very suitable for painting, without using special primers.  Very obvious this is no longer the case.

GM long acknowledged this problem and did not attempt to use the ultra low density or even normal low density SMC in the past for painted top surfaces.  What was needed was a suitable primer that would seal off the surface while filling in those pinholes.

With the seventh generation of Corvette, this urethane based primer is finally here but this new primer created a few new problems, problems that are directly related to the orange peel.  Ironically, the primer chosen might have not been the best choice since the new Corvette paint finish is likely to have the flaws the primer should eliminate.

Since this primer is thicker, it is rather difficult to apply in a smooth coat, the surface tends to be very bumpy and lumpy and ideally should be sanded prior to applying the color coat but... this is GM, cheap is the name of the game and after very thin coats of base and clear are applied, the orange peel in the primer can be easily seen.

What would it take to improve the paint quality on the newest Stingray and upcoming Z06?  GM would have to admit the problem and revise the primer or its application methods but this would be a miracle, impossible to happen.

The biggest irony of the newest Corvette is the fact that the final products, both the Stingray and Z06 ended up being true porkers, their weight, considering they lack coil over suspension, DCT and all wheel drive is truly obscene and revolting.  The price for this weight gain is a shitty paint quality that makes backyard rattle can paint jobs look terrific-certainly a triumph of (failed) technology, LOL.

Certainly, these are FACTS that the General Motors does not like to have surfaced but without disclosing this embarrassment, there will be no hope for improvement, especially since the typical C7 buyers prefer the ostrich approach, burring their heads deep in the sand and practicing ignorance and denial over a pint of freshly brewed kool aid.