I've been systematically cleaning the underside of my car as I have tackled mechanical projects. As other's have noted in the past, the 928 has a very comprehensive corrosion protection/undercoating system. My car has an off-white/gray mastic coating that is painted body color as a top coat...almost identical to my late 90's Audi A4.

While cleaning the 18 years of accumulated dirt, sand, grease, oil and grime off the underside, I noticed that many of the aluminum pieces (lower control arms, rear diff carrier and most of the aluminum suspension pieces as examples) seem to have a dark yellow, waxy coating. It cleans off fairly easily with a mild degreaser and medium-strength pressure wash. It is quite possible that this waxy coating covers the entire bottom of the car, but it would be very difficult to see against a black car (as mine is). I do not think it does...just the aluminum components as best I can tell.

This got me thinking. Is this waxy coating real or just the result of years of goop that has congealed on the aluminum parts? If real, was this applied at the factory or after delivery? I am amazed at the condition of the underside of my car (very good) and think this coating is primarily responsible for it. I'm now thinking it might be worth re-applying something like it when most of the major mechanical work is done. My car is only driven in the summer...no real risk of rust or corrosion from salt and prolonged dampness.

Anyone have any definitive knowledge on this? If it was factory applied, can something like it be sourced in the aftermarket? Has anyone else re-applied some form of similar protection? Several well-known rust proofers have a wax-like underbody coating that would be somewhat similar in nature. While effective, I'm not a fan of using these products on any of my cars, but I am open to the idea.

Regards,
SteveCo in St. John's

 

I'm not 100 % sure, but I think it was applied to the aluminum to prevent it from oxidizing. Reno

 

It certainly is a factory coating, it is basically designed for the extremes the 928 would see, like salted roads. I haven't seen any badly corroded local pieces here, but I have seen corroded English cars. The salt can get in crevices on the control arms and over time they can corrode.

Wurth would have supplied the majority of products at the time. Try an underbody wax. I cleaned up a lot of my car because I had to paint underneath it due to some forklift damage, but I reapplied the wax and other coatings where neccesary.

 

Thanks for confirming my thoughts. I, too, figured this was an effort to protect the aluminum from oxidation. I'll check with my friendly Wurth rep and see if they have something suitable...I'm sure they do. I need some Wurth shutz anyway to fix some shop lift damage spots, so I'll just add the aluminum protector to the list.

Regards,
SteveCo in St. John's

 

Hi Steve,

I just spent a great deal of time getting undercoating off of wheel well liners. What a mess,... sanding, mineral spirits, scotch brite, etc. They are only held on by a few fasteners. So, you may want to remove them before undercoating. They look much better now as black and smooth.

My mechanic saw the hack undercoating job, and said that it should not have ever been applied. He said that the original could last a lifetime. You may not need to reapply to most places. If you see bare metal that was previously coated, do coat that. Wurth products do rock. Could be the original product.

 

Thom;
I did the same as you...removed the fender liners and cleaned them up. They look great now, as you noted. FYI, I found lacquer thinner works wonders on these parts...removes paint overspray and greasy residue even easier than paint thinner, plus has the bonus of evaporating 100%. No need to wash off the thinner. Scotchbrite pads were a godsend in this activity as well.

I agree with your mechanic, there is no need for aftermarket undercoating on these cars...particularly if the vehicle will not see winter driving. I opted not to get my Audi A4 "undercoated", as is the norm here is the frozen North, for just this reason. It has essentially the same factory corrosion resistance defenses as the 928 does, plus an internal cavity wax coating to boot. I'm not worried about the 928 rusting away, but I would still like to ensure that the parts that need protection get it.

I have a few spots in the factory undercoating that need to be "repaired"...mostly due to errant shop lifts attaching to the wrong places. My frame front rails are bent a little bit, like virtually every other 928 I have seen. I plan to repair this damage over the winter and then apply the factory-look undercoating that has peeled of in these sections. I have even thought of printing up some big stickers that say "No Lift Here" like you see on aircraft! There always one idiot in a garage somewhere that will look to take the easy way out...and it will be just the time I am not there to make sure they do it right!

In all, the bottom of my 18 year old 928 look better than my 5 year A4! I think I will try something on the aluminum parts...if only as a test. I'll let the board know what I find.

Regards,
SteveCo in St. John's

 

Hi Steve,

Thanks for the tip on the lacquer thinner. That will save some time if I do the rear liners.

Here is a source for some Wurth products that might be the right stuff for the steel. See high build underbody spray and clean solve.

Let me know if you find a good aluminum product.

Also see this site for tips and products: http://www.autopia-carcare.com/ende.html


http://www.properautocare.com/wurhigbuilun.html

Respray wheel wells to a factory new appearance. Wurth High Build Underbody Spray produces a jet black, textured finish in wheel wells and on lower rocker panels. This spray will hide a multitude of blemishes and make old wheel wells look factory new. Underbody Spray provides permanent yet flexible protection against flying stones, rust, water and road salts. Before using, scrub wheel wells with a degreaser such as Amazing Roll Off, P21S Total Auto Wash or Wurth Clean Solve. Rinse and allow to dry thoroughly. Mask off areas you don’t want covered. Spray on Underbody Spray and wait 40 minutes before driving the vehicle. One can will allow two thin coats on all four wheel wells. Please Note: Federal regulations prohibit the shipment of aerosol products by air. This item is shipped by UPS Ground and only within the 48 connected United States. Sorry, we can not offer express 2 day or 3 day delivery on this product nor can we ship outside of the 48 connected United States. Made in Germany. Net wt. 19.5 oz. aerosol.8909071 $10.95

http://store.yahoo.com/classic-motoring/wuclae.html

Remove grease, tar and adhesives...from almost anything! Wurth Clean-Solve quickly and easily removes paint overspray, adhesive residues, tar, grease and marine scum. What makes Clean-Solve unique is that it can be used on all gel coat and Fiberglas finishes, acrylic finishes, aluminum, most plastics, vinyl. rubber and a variety of other surfaces. I always test a less noticeable area for color fastness when cleaning interior vinyl and plastics for the first time but I have never had this product harm any surface. And Clean-Solve works! I’ve used it to remove shoe scuff marks on vinyl side panels and heavy grease on exterior gel coat. If I need to clean and degrease anything, this is the product I reach for first. Please Note: Federal regulations prohibit the shipment of aerosol products by air. This item is shipped by UPS Ground and only within the 48 connected United States. Sorry, we can not offer express 2 day or 3 day delivery on this product nor can we ship outside of the 48 connected United States. Made in Germany. Net wt. 17 oz. aerosol.8909380 $9.95
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