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  #1  
Old 04-15-2003, 05:33 AM
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Join Date: Jun 2002
Location: Melbourne, Australia
Posts: 80
Angry rusting...looks like chicken feet

Got 10 spots on the roof around the sunroof, maybe caused by flying stones. A slightly raised centre with veins going out in all directions from this spot. Waited 1 year to see if they will grow larger and found they have now. Have scraped the paint to metal and treated with rust buster and wonder if anyone have this happenning too. Mine is a Euro C180 with original paint as far as I know. Have fixed up with original colour paint filling up the depression caused by the scraping and sealing with polish and wax. Will this last for a few years? A roof repaint was quoted at about A$450 on the cheap side and A$1500 on the killing end. There is no damage to the roof metal skin i.e. no rust through or holes.
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  #2  
Old 04-15-2003, 10:07 AM
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Join Date: Jun 2002
Location: Motor City, MI
Posts: 2,791
Just a technical aside: the corrosion veining phenomenon is called "filiform corrosion".
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  #3  
Old 04-15-2003, 06:00 PM
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Join Date: Jun 2002
Location: Melbourne, Australia
Posts: 80
Unhappy rusting roof panel

Kestas.
Thanks for your note about the rusting phenomenon. Any ideas how this is caused or why it occurs? Could it be the beginnings of rust coming through from under the panel due to the moisture trapped in the sunroof surround? If so, wouldn't it have holes already? This could be a really bad one. What about stonechips left ignored for sometime? The paint layer could have been fractured by very minute cracks which allowed water/moisture to seep into the sheet metal causing surface rusting undermining the surrounding areas as the trapped moisture have nowhere to go. Someone said it could be a bad repaint job or too thick paint used. I thought that Mercs have rustproofing or some kind of zinc plating or rust treatment to all panels considering the prices we pay for these cars. Your ideas will be welcome, thanks.
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  #4  
Old 04-15-2003, 08:39 PM
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Join Date: Jun 2002
Location: Motor City, MI
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Hocky1, you pretty much covered it. Generally speaking, you will not get corrosion on a panel that is sealed. It takes a breach in the surface, such as a stone chip or crack in the paint to start corrosion. The protective coating is gone and the steel is open to the elements.

At the risk of getting too technical, any type of crevice on a micro scale will concentrate the corrosive solution (or electrolyte) and bring on corrosion. This pretty much describes the vast majority of automotive corrosion. Filiform corrosion is a variation of this abovementioned phenomenon. It is corrosion that burrows like a mole under a coating. I don't know the reason why it acts in only one dimension.

Zinc coatings are only a stop gap measure... they don't last forever. Zinc sacrifices itself to protect the steel. When it's gone the steel begins rusting.

This is why it's so important to cover up any stone chips.

A good protective paint job on steel consists of the following:
- Zinc coat
- phosphate coat
- primer (e-coat is best)
- topcoat (colorcoat and clearcoat)

It is hard for repair jobs to equal factory paint jobs for a variety of reasons. Lack of phosphate and e-coat is some of the problem. Another is the lack of zinc coat from feather-grinding during body work.
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