What makes paint bubble?

[sander]

Why does rust underneath the paint make it bubble?

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'71 911E with Webers
Golden Gate Region PCA
http://www.geocities.com/edrecinto/

[Swank911]

actually, you might ask: What causes the rust under the paint?

I have the same problem, bubbles here and there, but a little light sanding just shows a pock mark.

[sander]

I'm going to showcase my ignorance here, but I thought the bubbles always meant rust underneath - but maybe I've got that wrong?

Does paint bubble for some other reason aside from rust?

And that is a good question, Swank, how does the rust get under there in the first place?

[Early_S_Man]

No, it isn't always rust ... if, for example, the paint job is recent -- one year old, or less. It can be the result of moisture in the lines of the air supply hose to the paint gun when the topcoat was applied. And, it doesn't show up immediately after drying, either!

It can also occur because of contaminants on the primer if there was significant delay between primer and topcoat application!

Likewise, it can also be the result of inadequate cleaning and preparation of old paint, if shortcuts were taken.

Oh yes, there is also a malady unique to basecoat/clearcoat repaint jobs that shows up three-to-five years afterwards that I call the fish scale syndrome -- large patches 3-10 inches across, or larger that starts off as a few blisters in the clearcoat, and gradually grows to a uniform fish scale pattern just like a largemouth bass has! And, no, I don't know what caused it, because it was a professionally-done paint job in both of the cases I have experienced personally! Eventually, the clear coat flakes off in very small patches after repeated wand washes over a couple of years, revealing smooth basecoat underneath!
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Warren Hall
1973 911S Targa

[This message has been edited by Early_S_Man (edited 06-06-2001).]

[MMBRAZIL]

If the bubbles are in the fender (specialy the rear), it could be the tire rubbing and heating up the sheet metal....that causes bubbles also.

[dhamilto]

Wierd stuff has recently started happening to my SC. Perfectly round bubbles, some as large as a dime, are appearing in various places, especially at the bottom of the doors jams. I popped a few of theses and they were full of water. On closer inspection, the water appears to be getting in from behind the door catches where the underseal is broken (inside the rear wheel arches below the Targa bar). The wierd thing is - how the heck can it force such a big bubble to appear. There is no rust underneath it until the water got in. Any takers? The paint was pro done 5 years ago.

83SC.

[Norm Faustino]

A lot of self proclaimed paint pros use little more than standard prep work and reap the profit hoping that the imperfections wont show up for a while, the fact is the prep work is always the cornerstone of a concours quality paint job and to answer your question about what allows water under the paint is that if any sanding is done to the paint that exposes bare metal you MUST!! use an etching primer to prevent the occurence of delamination where the paint actully just sits there on that one spot holding on by its edges and the surrounding paint that is bound to the body the resulting gap, microscopic as it may be will allow condensation to form under the paint and lift it up, causing "bubbles" to form this condition is even more prone to happen to galvanized metal such as on the Porsche.
My Unqualified professional opinion!!

[CamB]

I understand from my paint guy that any sort of grease (especially silicone) can cause the fish thing.

This can be as simple as grease from the hand of the painter during the prep...

In fact, everything I have been told suggests that preparation is key, especially that etch primer bit. In particular, I believe the current thinking is that filler should go over etch primer rather than onto bare metal as filler is porous...

Cam

[This message has been edited by CamB (edited 06-06-2001).]

[Jens Wendorff]

One other possible cause for small bubbles is zinc-coating. No, not the factory's original, but I heard of more than one people (also at least one on this board) who have zinc coated their cars following a bare metal strip. They thought to have eliminated all rust problems- but after a couple of years the paint comes up in small bubbles.
Cant remember the exact reason, but still have the picture in my mind

jens