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Undercoating Removal Nightmare

Pelican Technical Article:

Undercoating Removal Nightmare

Mitchell Sam Rossi


10 hours10 hrs






chassis dolly

Applicable Models:

Porsche 911 (1965-89)
Porsche 930 Turbo (1976-89)
Porsche 964 Carrera 4 (1989)

Parts Required:

Stripping tools, sand blaster

Hot Tip:

This is more work than you think.

Performance Gain:

Lighter weight for racing.

Undercoating Nightmare


This article was originally published in HPBooks' Porsche 911 Performance: Building the Ultimate 911 for High Performance Street or Road Racing, by . Rossi is a contributor to European Car, Road & Track, Sport Compact Car and Los Angeles Times. As part of our selection of vintage 911 must-reads, this book provides a comprehensive look into the high-performance modification of 911's for the street and track. Covering topics ranging from engine building to chassis strengthening, this 144-page guide will certainly take your venerable 911 to the next level of performance. Click here to order this book.

I now know Sisyphus's misery. Having cheated death, as Greek legend states, the Corinthian King was sentenced to eternally push an enormous boulder up a hill only to have it slip from his grasp as he neared the summit. A cruel, agonizing struggle he was never to overcome.

I know this torment, or as near as I can imagine after having tried my hand at removing the undercoating from my S. To be certain, like the Tartarus stone, this horrific material is bewitched. A vile black scourge to anyone building a race car or looking to restore a 911 to extreme concours condition.

Figure Figure 1: An example of the 911's thick rubbery hide.

Figure Figure 2: Like its underbelly, the 911's interior floor panels were given a good thick layer of coating.

There is little argument that this coating offers a huge advantage to automobiles operated under normal circumstances. Firstly, it is a defense against highway noise where the ceaseless assault of road debris would pound the undercarriage until the passengers were rendered deaf. The thick rubberized skin also protects the car's painted surfaces from becoming pitted and scored.

The early 911, like most vehicles from that era, had yet to benefit from the automotive primers and metal treatments employed today. Any compromise in the paint lead directly to the blight of corrosion, especially in geographical regions that used deicing salts on their roads.

Figure Figure 3: During its travels, the S made good use of its protective undercoating - from an outing in the snow to a spin on the beach.

Figure Figure 4:

As Porsche took great pride in the longevity of their powertrains, they have always been compelled to insure their car bodies last as long. As one factory brochure from the early 1970s stated, at least thirty-five pounds of the protective material was applied by brush to the 911's underside.

In 1971, the factory made further inroads with respect to anti-corrosion by adopting a galvanized floor pan. Galvanizing uses electrolysis to coat either steel or iron with zinc which hinders corrosion. Porsche utilized this process for the 911's entire chassis in 1976.

For a street car, undercoating is a must. The S, however, was slated to spend ninety percent of its life on the track and away from the corrosive elements. Of course, there would be Sunday afternoon outings along California's Pacific Coast Highway and an occasional autocross event that could be attended without the ensemble of trailer, tow vehicle and the mass of equipment required for high-speed competition.

By eliminating the protective seal these jaunts on public highways would have to be planned with an eye on the weather. The last thing one wants to do is be caught in a downpour in a car with no undercoating, no heater, plastic windows and competition brake pads, which by definition, need a good amount of heat to work properly.

From the aspect of building a track car, the negative side of undercoating is simply its weight. While the factory might have stated there was thirty-five pounds under the car, they failed to mention the amount inside the passenger compartment, trunk and engine bay. After inspecting the interior, it is not a stretch of the imagination to guess there is another twenty pounds of this sealant in the cockpit alone.

While the newly built 2.7-liter motor was waiting, I disassembled the car once again. But unlike Phase I, this time everything was stripped from the chassis; the brake and suspension system, fuel system, interior...everything. To keep the chassis mobile in my home garage, it was bolted to a custom-built four-wheel dolly.

Figure Figure 5: A chassis dolly is a simple but important tool for this type of project.

Shucking the rubberized husk from the chassis seemed only to be a matter of perseverance and the proper tools. With hindsight, my naiveté about this step in the project still makes me cringe.

Equipped with a 25 gallon, six horsepower, single stage air compressor, a half-dozen wire wheel brushes and an armory of grinding tools, I donned an old shirt, jeans, hat, paper mask and a full-face safety shield. Slipping under the right rear fender, I began my crusade.

After four hours, the weight of the air tool had quadrupled. The wire brushes had shed most of their brass quills, which, accelerated by their whirling velocity, had pierced my shirt and penetrated my skin. What little undercoating I managed to clean, had transformed into a gray powdery snow that covered me from head to foot. And I had only cleaned a small vertical panel of steel.

Figure Figure 6: Attacking the undercoating inch by inch.

The next day, I employed small cup-type wire wheels that could reach into the crevices between the fender and chassis. Six hours later and the right rear fender was nearly, but not completely, void of undercoating. The following weekend was spent inside the left fender well.

With progress painfully slow, a fellow racer suggested another tactic. It involved a propane torch and a stiff bladed spatula. Needless to say, this is not a technique I would recommend. It was not only as tedious as the wire brush, but was both messy and a shade on the dangerous side. The thirty year old coating, I quickly discovered, can burn on its own for several minutes.

Figure Figure 7: Hand tools are futile against the undercoating.

Still, I was not willing to admit defeat. At the arrival of the fourth weekend, I had managed to find raw metal under two rear fenders and a minuscule corner of the engine compartment. But I had a new weapon in my arsenal. The Würth DBS 3500 pneumatic wire brush.

Figure Figure 8: My arsenal of stripping tools.

Figure Figure 9: The extreme cold of dry ice useless.

Outfitted with talon-like spikes embedded into a circular belt, this cumbersome, two-handed contraption gave me new hope. And, indeed, it clawed the undercoating with voracity. Inch by inch, the black tide began to retreat under the assault.

But almost as soon as it started, the Würth began to slow and falter. In the far corner of the garage, the air compressor roared as it tried to keep pace. It could not. Designed for the professional automotive restoration shop, the swirling claws required a larger air compressor capacity than the home unit could offer. Try as it might, there was not enough pressure to keep the belt spinning. The Greek boulder had slipped from my hands and was tumbling down the slope.

I was still not willing to give up but instead turned my attention to the inside of the car. Unlike its rubberized belly, the interior floor pan was covered by a hard, waxy material that could be chipped free with a hammer and chisel.

Several dependable sources had suggested covering the floor with dry ice and allowing the material to become hard and brittle. I was told it would then simply break free in large black sheets. Is anyone selling the Brooklyn Bridge? Because if you buy this dry ice fable, you'll buy the bridge too. I know I did.

A block of dry ice set directly on the floor did not work. Nor did a block of dry ice broken up and spread across the floor. Sure, driven to near sub-zero temperatures, the material splintered before the chisel blade, but it didn't peel off in sheets. The chisel and hammer required more effort than the air tools and now my fingers were freezing as well. The boulder, it seemed, was heading down the hill again.

Figure Figure 10

Figure Figure 11: The rubber coating gives way under a blizzard of sand.

Finally, taking into account the time and effort I had spent removing minuscule amounts of the car's leathery hide, I felt I had only two options left. The slash-n-burn of chassis preparation...sandblasting. Or acid stripping.

With acid stripping, the car is unclad by an acidic solution but this involved having the entire chassis dipped in a huge tank. While the owner of the stripping shop assured me it was a harmless procedure, I had misgivings. If the corrosive chemicals used in stripping are not completely flushed from the chassis, it continues to eat at the car from within its hidden corners.

High pressure blasting, on the other hand, sends sand into every crevice of the car and can often be found shaking loose years after the car is finished and on the road. While sand would be an annoyance, it seemed a better alternative than acid residue.

To my surprise, the first few companies I contacted about sandblasting the chassis would not consider doing the work. They had been confronted by Porsche's resilient undercoating in the past. They hung up on me. Luckily, I was referred to Western Shop Blasting in Santa Ana, California.

Western offered several types of blasting compounds, including, sand, glass bead, plastic medium, walnut shell and aluminum oxide. The manager explained that each of these compounds had a particular application. Walnut is best used to remove paint, but not rust or undercoating. Glass bead for fine parts, aluminum oxide for aluminum and wood. Contrary to expectations, plastic medium imparts a rough finish.

Figure Figure 12

For the 911S there was only one option, the abrasive, hard hitting sand delivered under high pressure. Because of the air pressure levels used, there was concern that the un-reinforced sheet metal would distort. Thus the paint on the Porsche's roof was not blasted clean.

It was estimated that the job would take the shop's crew about five hours to complete. It ultimately took ten hours. I felt vindicated. These were professionals with all the right equipment and experience. But vindicated or not, I was finally rid of the rubberized curse.

The next step, and one taken quickly, was to seal the car with a coat of temporary primer. Even in Southern California bare steel oxidizes instantly.

While the early 911 is renowned for its rust problems, it is not from Porsche's indifference. Clearly the factory tried to address the problems as, thirty years later, removing the undercoating is no easy task. It requires an infinite amount of patience, an excess of spare time, and a penchant for self-abuse. This is not a labor of love. It is an unyielding torment and offered a choice, the next time I'd rather try my hand at Sisyphus's boulder and hill.

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Figure 1

An example of the 911's thick rubbery hide.

Figure 2

Like its underbelly, the 911's interior floor panels were given a good thick layer of coating.

Figure 3

During its travels, the S made good use of its protective undercoating - from an outing in the snow

Figure 4

to a spin on the beach.

Figure 5

A chassis dolly is a simple but important tool for this type of project.

Figure 6

Attacking the undercoating inch by inch.

Figure 7

Hand tools are futile against the undercoating.

Figure 8

My arsenal of stripping tools.

Figure 9

The extreme cold of dry ice useless.

Figure 10

Propping it up on its side

Figure 11

The rubber coating gives way under a blizzard of sand.

Figure 12

Success at last!

Comments and Suggestions:
Wildmancar! Comments: Undercoat Removal, difficult at best & very nasty!
Tried the needle scaler, air chisel, wire brush and hand held scraper.
Also used a hot air gun, heating up exterior of fenders, while a friend scrapped underside of fender with a sharp putty knife, worked great!
Also used a long flat blade screw driver and small hammer to break loose undercoating in deep pockets.
Good luck to all of you amateur restorers!
August 20, 2014
  Followup from the Pelican Staff: Thanks for the input on this one. We appreciate the help.
- Nick at Pelican Parts
911bus Comments: You guys are wasting your time scraping and picking away at undercoating. I used to do that until I worked out this method
For the at home do it yourself guy, buy some of the canned compressed air. The stuff at Costco etc for cleaning cameras and key boards. Shake it well, turn upside down and spray in a small area aprox 4-6" take a rubber mallet and give it a tap, if it dosen't fall off right away use a plastic tool like a windsheild seal tool or trim tool to start an area then repeat with the compressed air. You should start getting large pc's off quickly without damaging the paint underneath. When the can is sprayed upside down it becomes Liquid nitrogen and freezes the undercoating allowing it to freeze and fall off.
Works great! Buy lot's of cans!
November 18, 2013
  Followup from the Pelican Staff: Thanks for the additional Info. We appreciate it.
- Nick at Pelican Parts
gere Comments: I am working on a Volvo 1800ES and have found any EASY method. I use an air chisel with a semi dull edge. I follow up with a needle scaler on the thinner areas. A wash with rag soaked with mineral spirits gets the rest and wiping with clean rags finishes it up ready to sand and paint.
March 15, 2010
  Followup from the Pelican Staff: Thanks for the additional info. We appreciate it. - Nick at Pelican Parts  

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