Paint is reacting

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PostPosted: Tue May 30, 2023 7:36 am
Am going to strip and paint my car (1980 Porsche 911 Targa). Before I do, there's a problem that needs an answer. Please have a look at this...

01 Hood Bad Paint Silver.jpg


Entire car was last painted by a shop around 2005. Front hood (section seen above) was done again in 2012 because the paint was shrinking and so pulling up from the crevices that lead to the fresh air intake. That redo was done by a different shop than the first. Fresh, all the paint on both occasions looked great. Over time (years), what you see above has appeared in different areas around the entire car. Most of this reacted paint is on up facing areas of the car---front hood, rear hood and tops of the rear quarters. No painted roof as this is a Targa. The only place it is on a side (currently) is the left rear quarter panel. The car is parked indoors so sunlight would seem a minor player. Before the hood was redone, there was just a faint appearance of what's seen in the photo above on the hood---no where else at the time. I discussed it with the "redo" shop and their response was to apply a coat of zinc chromate. This was done and hood sat for a few days then they sanded the zinc chromate off. Then primed and painted the hood to a good looking finish. Eventually what you see now arose in a very slow progression over years, again, over different areas of the car.

Of course I don’t want to start redoing the car before knowing what's happening and more importantly, how to quench it so there's no resurfacing.

I did not see the car in paint progress when it was originally completely done. I did see the front hood in progress...

006 Hood to Bare Metal.jpg

007 Zincromate Hood.jpg

010 Zinc Removed 2.jpg

011 Finished Hood.jpg


Guidance with this would be much appreciated. This is my first post so I do not know what protocols exist if any for newbs---please forgive if I fumble my way through this.

Karl ~~~



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PostPosted: Tue May 30, 2023 3:56 pm
It looks to me like either solvent that was trapped below the clear or an incompatible primer/paint.

Knowing what materials (primer, base, clear) would help. Also I am not sure how Zinc Chromate fits into the scheme of things. Hopefully one of the pros will be able to assess the situation your in better than I can.
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PostPosted: Tue May 30, 2023 5:24 pm
'68 Coronet R/T wrote:It looks to me like either solvent that was trapped below the clear or an incompatible primer/paint.

Knowing what materials (primer, base, clear) would help. Also I am not sure how Zinc Chromate fits into the scheme of things. Hopefully one of the pros will be able to assess the situation your in better than I can.


Thank you '68. A return visit... after all was said & done at both shops, I departed not too pleased with management in both cases. Paint work aside, there was BS after BS to deal with. Missing parts, things not working, incorrect assembly, overspray. Outright sloppy handling and lots of excuses. Going back now, I doubt they'd remember work done years ago. And I have no reason to trust either shop.

Was advised to me that zinc chromate would "improve the condition of the metal" should there be something "wrong" with it. Thinking the shop manager would know his stuff, I didn't question his recommendation or their process with it.

What seems important here is the same reaction is occurring to the work that both paint shops did to the car. And notably years apart. While both shops could have screwed up their work resulting in exactly the same reaction, what are the odds?! I'd agree there is a possibility of this happening. I can't stop thinking there's something in the metal?

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PostPosted: Tue May 30, 2023 6:10 pm
Normally, I would strip to bare metal and then apply two coats of epoxy primer.
I have always had nothing be trouble with metal "treatments".
1968 Coronet R/T


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PostPosted: Tue May 30, 2023 6:52 pm
The damage looks to me like an everyday fry up. not unusual if the colour coat is sprayed on too wet over a substrate that could be a bit solvent sensitive. Can't believe that anyone would let that out the door, though.

Zinc chromate (and similar products) is old technology. These days a quality epoxy will do a better job, sometimes with some pre-treatment for aluminium or plastic panels.
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PostPosted: Tue May 30, 2023 7:44 pm
Being an aircraft mechanic I am experienced with coatings on Aluminum.
And Yes It should be treated very differently than Steel. Including metal treatment and specialized products for painting including Zinc chromate Epoxy.

Bare Aluminum starts the corrosion process the second it is exposed, Sanding of Aluminum should be avoided in most cases as it damages Aluminum Cladding which is a Micro coating of Pure Aluminum on the surface of Any aluminum piece.
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PostPosted: Wed May 31, 2023 9:31 am
Doright wrote:Being an aircraft mechanic I am experienced with coatings on Aluminum.
And Yes It should be treated very differently than Steel. Including metal treatment and specialized products for painting including Zinc chromate Epoxy.

Bare Aluminum starts the corrosion process the second it is exposed, Sanding of Aluminum should be avoided in most cases as it damages Aluminum Cladding which is a Micro coating of Pure Aluminum on the surface of Any aluminum piece.


He never mentioned this was aluminum that I saw.
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PostPosted: Wed May 31, 2023 1:55 pm
'68 Coronet R/T wrote:
Doright wrote:Being an aircraft mechanic I am experienced with coatings on Aluminum.
And Yes It should be treated very differently than Steel. Including metal treatment and specialized products for painting including Zinc chromate Epoxy.

Bare Aluminum starts the corrosion process the second it is exposed, Sanding of Aluminum should be avoided in most cases as it damages Aluminum Cladding which is a Micro coating of Pure Aluminum on the surface of Any aluminum piece.


He never mentioned this was aluminum that I saw.


Not aluminum. Is galvanized steel.



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PostPosted: Wed May 31, 2023 2:01 pm
'68 Coronet R/T wrote:Normally, I would strip to bare metal and then apply two coats of epoxy primer.
I have always had nothing be trouble with metal "treatments".


68... From your input, I gather an epoxy primer would seal in whatever "might" be in the metal causing the paint to react.



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PostPosted: Wed May 31, 2023 4:05 pm
'68 Coronet R/T wrote:
Doright wrote:Being an aircraft mechanic I am experienced with coatings on Aluminum.
And Yes It should be treated very differently than Steel. Including metal treatment and specialized products for painting including Zinc chromate Epoxy.

Bare Aluminum starts the corrosion process the second it is exposed, Sanding of Aluminum should be avoided in most cases as it damages Aluminum Cladding which is a Micro coating of Pure Aluminum on the surface of Any aluminum piece.


He never mentioned this was aluminum that I saw.

My Bad I saw Zinc Chromate and Jumped thought it was some sort of special Aluminum Porsche ? I never seen anyone use Zinc Chromate on steel b4.
If its steel I would do what you said Sand it strip it Epoxy it move on to primer, base coat & clear.
Dennis B.
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