Factory undercoating on a '72

[clear911]

I have followed this post with some interest.

My ’72 E built in Feb ’72 is just about ready for the color coat. While not a pristine example, it did have the original paint beneath an earlier repaint. While removing the paint to bare metal this is my experience:

1)Underbody – The underbody coating was not removed, so I can’t really say much here. Appears to contain some body colored overspray beneath accumulated dirt associated with years of use.

2)Rockers - The rockers were coated with a factory applied tan/beige material having a fairly coarse texture that was rock hard on top of a light colored seal primer applied to the metal. No external corrosion.

3)Wheelwells - Appears to be a brushed on satin black paint with occasional body color bleed thru. Not sure what underbody coating material was applied here.

4)Front and rear bumpers - A hydrocarbon based (bituminous) undercoating which resulted in substantial ricochet when bead blasted and a tarry mess when paint stripper was applied. I do not know if this was original or not. Given the ease of removal of the bumpers and the material applied possibly not, although the rear bumpers based on other used examples from earlier year model LWB & SWB models could have original undercoating material. On the steel “S option” bumper, a minor amount of surface rust was present beneath the undercoating which may have become an issue over the long term.

5)Underneath the hood, between the stiffening braces - Factory applied tan/beige material having a fairly smooth texture on top of a light colored seal primer applied to the metal. Several coats of paint stripper was required to get to bare metal. Tough stuff. No rust here.

The rocker (and presumably underbody coating) material and the underhood material matched as Dr. Johnson indicated they would for LWB cars in his book. See page 6. Supposedly a PVC type material, this would be consistent with the rockers and underhood material that was removed.
So what am I going to apply prior to the color coat? These are the options that I have considered:

1)Wurth SKS Stone Guard (gray) – A medium gray material that I do not believe is the right stuff. According to the MSDS this contains primarily talc. A water based product, it takes 3-4 days to fully cure. Maybe longer if humidity is high. Requires a special Schutz gun to apply properly. My painter believes painting on top of this is risky business. I have two (2) 30 oz. bottles if anyone wishes to give it a try, one unopened, one slightly used.

2)Wurth Underbody Seal (beige) – According to the MSDS contains a Magnesium-Silica talc (25-35%) an ethenyl benzene polymer w/1,3 butadiene (5-7%) and quaternary ammonia compounds (3-5%) with titanium dioxide (1.5-2%) white paint base. Uses naptha/tolulene solvent.

3)3M Rocker Panel Spray (tan) P/N 05910 - Comes in a spray can. According to the MSDS contains vinyl chloride-vinyl acetate-maleic acid polymer (5-10%), the ubiquitous talc (3-7%) and silica (0.5-1.5%) with ketone solvents.

4)3m Underbody Seal (beige) P/N00874 - Uses a regular rocker Schutz gun to apply. According to the MSDS contains Acrylonitrile-1,3 Butadiene- divinyl Benzene copolymer (5-10%), talc (3-7%), silica in the form of glass bubbles (1-5%) and a butyl phenol-formaldehyde resin (1-5%) in a ketone/toluene solvent.

As can be seen, the bottom three options are worthy of consideration due to the residue materials and the volatile solvents which should give quick cure times. The factory surely would have taken this into consideration just as a custom painter will. It is questionable whether any of these compounds match what the factory used back in the day. As technology changes with time, original materials (particularly complex materials such as these) cease to exist. Thus the saying it is only original once and can’t be duplicated.

So what am I going to use? While I think the 3M Rocker Panel Spray (tan) P/N 05910 may be the closest match to factory if Dr. Johnson’s observations are considered, I am going to leave it up to my painter and his experience with these products with longjevity in mind. Since he has a great deal of experience with 3M it will likely be the 3m Underbody Seal (beige) P/N00874 so the correct texture can be applied underhood with the Schutz gun. Besides, I kind of like the idea of resin holding this stuff together and forming an additional chemical bond on the car.

Robert

[tfiv]

tom,
i agree the bottom is not black but is the bottom body color?

The center of the floor is the sort of natural color of the undercoat. There also appears to be some staining from maybe cosmoline? It appears the goal was only to cover what you might see from the side of the car. The silver overspray is covered by the black.

[tfiv]

Is this 72 100% original and never restored , never received any touch-up's ?

Yes that is how the car came to me. I've since done a very careful re-paint. I walnut shell blasted the exterior to just below where the rocker moulding starts. I even preserved the undercoating on the underside of the fender lips. I intentionally left all of the grime on the undercarriage until the paint was done, so accidental overspray wouldn't stick. I have not touched up the underside of the car in any way. It took the better part of a day cleaning cleaning to get this result. - yuck.

[robmog]

robert, i used the wurth beige undercoat.
its a perfect texture match for the original.
when dry (fast) it is pretty hard also like original.
heres a (too small)picture of my trunk.
the stoneguard had too fine a texture and looked wrong.

[clear911]

Bob,
Beautiful looking work.

The Wurth underbody seal looks darker than I would have guessed. Appears to be almost the color of the 3939 Olive that my car is. The Wurth products are difficult to find in the part of the country where I live which is why my painter is more familiar with 3M products. He is going to spray a proof card showing that he can match the texture of the rockers and the underhood. The texture is most important to me.

From a technical point of view the Wurth Underbody Seal (beige), 3M Rocker Panel Spray (tan) P/N 05910, and 3M Underbody Seal (beige) P/N00874 products are essentially similar based on the compositions given in the Material Safety Data Sheet (MSDS). Solids material are slightly different as are the materials holding them together and the solvents preventing them from setting up in the can, but all in all about the same. The keys setting these products apart from the rest are the quick dry times and the hard consistency at cure, which I think are important.

Funny thing is, only the 3M Rocker Panel Spray (tan) P/N 05910 contains a Poly Vinyl Chloride (PVC) compound (vinyl chloride-vinyl acetate-maleic acid polymer (5-10%)) such as Dr. Johnson referred to in his book. It is also a very light beige color, similar to what I removed.

Perhaps I can get a few spray cans and cut the top off and shoot it thru the Schutz gun and achieve the proper texture for the underhood undercoating which had sort of a krinkly appearance, like too thickly applied paint blown with air as it dried.

The issue I am having difficulty with is coating the backside of the bumpers with underbody coating. I guess I’ll do it, but it just seems like extra weight and something that may cause a longjevity/corrosion issue down the road.

Robert

[Tom F.]

Bob,

The issue I am having difficulty with is coating the backside of the bumpers with underbody coating. I guess I’ll do it, but it just seems like extra weight and something that may cause a longjevity/corrosion issue down the road.

Robert

Porsche did not put any underbody coating inside the rear bumpers, at first. I am as sure as I can be that 1969 cars did not have it, and that 1973 cars did. I have original specimens of both.

[robmog]

robert,
that is the color olive, i put in the picture to show the correct texture.
the beige is really very light tan in color.
i can post pictures of the unpainted bottom if you like, closeups too.
ps gotta love olive!

[clear911]

I agree with Tom F. from Massachusetts. The original 73 T that I have had a chance to look at had the undercoating on the backside of the bumpers while an original SWB bumper that I have seen did not. I am going to follow the other Tom F.’s (tfiv) example with my bumpers, just not with a hydrocarbon based underbody spray.

I believe my bumpers had been recoated with the hydrocarbon based material during the earlier repaint. This was a poor choice of material based on the surface corrosion evident once it was removed. Over time, the corrosion would have become a bigger issue than it was. I believe a PVC based or equivalent type material will give the best service.

Realistically, I believe the Wurth and the two 3M products likely offer similar performance and any of these products will be okay with me.

The product that I suspect is the PVC underseal for our cars is made by Henkel, Teroson Variodur Classic, which is light beige in color.

I cannot find an MSDS in English for the U.S. , so I don’t know if the product is sold in the U.S. or not. There is definitely a presence in Germany and the UK.

Teroson , based in Heidelberg, not too far from Stuttgart, was an old line brand dating back to the mid-1920’s that was established by Theodor Ross & Son specifically for the manufacture of automotive related products. Teroson would have been a known entity to Porsche. If Teroson was the original manufacturer of the underbody coating, I would theorize that business arrangements might extend to other automotive products such as the bitumen mat sound deadening on the floorboards.

If this is so, then:

1) The Henkel product Terodem-SP300 self-adhesive bitumin/plastic barrier panels which are flexible when heated may be a good choice for the floorboards for the suppression of road noise. I don’t know if they are the same as original, but I suspect they are close.

2) The “Cosmoline” underbody coating on the Ray Allen 700 mile 1973 911T (Excellence, August 2009, “The Allen T”), was apparently applied to the undercarriage of that car prior to leaving the factory as it was also to cars in later year models. Henkel’s Terotex-Wax is probably a good choice here, if it can be found. I’m not sure if the actual trade-mark Cosmoline manufactured by E.F. Houghton & Co. of the US was used back in the day, but it could have been.

I think it would be interesting to know what the Porsche Restoration Center uses for these applications.

Funny how I thought the color in Bob’s photo was Olive! I now recognize that color just about every time I see it. It is a really interesting color as it seemingly changes hue with lighting conditions. I think it is one of the best colors for the 911 when photographed. A skilled photographer can capture the essence of the early 911 and the period with this color. I have to agree, once you know it, you gotta love olive.

Robert

[Mudi]

Looks to me like it was undercoated then painted then grimed up over thirty years.. Bob

[clear911]

Looks sort of familiar. PVC type undercoating, then body color paint overspray followed by many years of road dirt, perhaps embedding in the waxy “Cosmoline” coating if it were never removed with the end result being a grimy appearance.

I think Porsche was very serious about rust-proofing the 911 which was, of course, a good deal more expensive than most other cars at the time. Plus considering the strict German vehicular laws prohibiting the operation of damaged or rusted out vehicles, it’s easy to see why Porsche began to zinc the 911 floorboards in 1970-71. Ultimately, I believe Porsche was the first manufacturer to completely zinc the entire vehicle, today’s norm, beginning in the mid-1970’s.

The “Cosmoline” coating I believe was used by Porsche as a treatment for the undercoating. From the product description of Henkel’s Terotex-Wax on their website, “Terotex-Wax is primarily used on vehicle underbodies, particularly for the treatment of all protective coatings, like those based on PVC, PVC/wax, bitumen/rubber and rubber/resins.”

I suspect most, if not all, the cars were wax coated, but only the original owners could say for sure.

I don’t know if the concours judges look for this or not, but probably something to consider.

Robert