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I'm currently in the process of restoring an older car (a 1988 BMW M5, in particular) after it sat in a driveway in the Pacific Northwest for ten years, waiting on an engine rebuild.

My paint restoration process more or less started with rubbing compound. After getting the initial fog off, we noticed these dots underneath it:

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They aren't the end of the world, but they're quite visible when light is shining on them.

They show up in clusters over the roof, and most clusters cover just a few square inches, but they seem pretty evenly distributed across the whole roof. There are probably ten such clusters.

I've only checked the roof so far, but it's my assumption that they'll show up all over the body of the car.

I assume they're caused by weather of one sort or another--it saw rain, sun, snow, you name it--but is there anything I can do to get them to go away? Or at least, anything to make them a bit smaller?

I'd live with them sooner than I'd get it painted, since it still looks good and there are better uses for that money on the car, but I'd like to make it look its best.


To the idea that it could be rust, that was definitely my initial reaction as well. That said, I thought it was weird that:

  • The dots are very hard, and there's no bubbling or paint chipping.
  • There's no bare metal or rust visual anywhere on the car, except some pretty aggressive stuff in the trunk, but that's far from this, and does have paint chipping and all that classic stuff.
  • The paint is in very good shape, with the exception of these markings. It's still the stock paint job, but it's all very smooth, with no damage.

That all said, I know rust is tricky, so it's 100% possible.


A few more images:

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More pictures on imgur...


So, following advice by a couple comments, I popped one of them open with my knife. It was brown inside, which seems very rust-like.

However, I'm still not convinced! The pattern is so strange for rust, and the paint's in such good shape with this exception.

I found the idea of sap really compelling--the car has been sitting under a tree for the better part of its time outside, and these "clusters" could have easily been caused by dripping. It also explains why I can't find any such marks on the doors, and potentially why there's only one on the bonnet (over which tree coverage was less).

As for the idea of it being clear-coat coming off, I'd believe that. Although the paint is in really awesome shape everywhere else (sorry to keep saying that--I really like this car), so it seems a bit curious that that would be a problem. Not to mention, it's not bubbling. I don't know if that's what clear-coat would do, but I'd assume so. I can't move the dots at all, and even pressing them with my knife felt like metal-on-(painted)-metal.

However, I can't ignore that what I found in that one was probably rust. Whether that was a cause or effect, though, I'm still not sure.

And for what it's worth, on the matter of it being rust, it can be really difficult to tell, but as far as I can, from getting close and looking, I'd really swear that these are under the paint, not on top. I can't think of how anything could have gotten under, without coming up from below (there's no sign of that from inside, but one never knows).


Listed above, but for those following along with edits, I've uploaded more photos to an album on imgur. Please don't hesitate to request more if anything would help! I'm only limited by my own lack of creativity--happy to take as many as might potentially be useful.

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    It looks like rust but hard to say from that one photo. Maybe other images would help. – Zaid Mar 24 '16 at 2:41
  • @Zaid I'll add some more pictures in a bit, thanks. For what it's worth, I don't expect them to show much more than that one does--it isn't very visually interesting, unfortunately, but I'll see what I can do. As for rust, that's definitely a good thought, but I've included a section at the bottom of my question with some more details (that I meant to include in the first place; sorry). – Matthew Haugen Mar 24 '16 at 5:09
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    I'm wondering if block wet sanding with a very non-aggressive (3000+ grit) sand paper would give you insight into what's going on. It would knock the tops off of the bubbles, whatever they are, and show you if it might be rust underneath, or if it's something lying on top of the paint. I say block sanding to keep the paper flat and only take off the top of whatever it is. If it is rust, it's going to require a repaint anyway. If it is some type of paint contaminant (something on top), it should become obvious quickly. Do this in the least conspicuous area. – Pᴀᴜʟsᴛᴇʀ2 Mar 24 '16 at 11:20
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    +1 for Paulster@ idea on checking but Id just "shave" the top off a couple spots with the tip of a blade, if its rust you will see it right away and it will be small enough to be covered the same way you would do with a stone chip if its not rust. Can't see from the pics but you might be able to find out something checking around the sunroof. The only similar looking thing I saw, that wasn't rust, was tree sap dripping on one car that sat under a tree for years, the stuff was as hard as the clearcoat, took forever to clean it off without damaging the paint, but hard to say just from pics – Erik vanDoren Mar 24 '16 at 14:56
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    Could the clear coat be separating from the paint? I've had a car do that years ago and it eventually (the clear coat) began separating from the paint in sheets but it did look similar in the early stages. – DucatiKiller Mar 24 '16 at 22:46
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It's more than likely the clear coat, and not rust. Here's why.

Temperature change is one factor. The clear coat can sometimes expand differently than the base coat. Areas where snow get piled on can be a factor, as well as sections that get heated frequently, ie. hood.

Acid rain can breakdown the chemical bonds, and the above separation can occur.

"Solvent popping", this typically does not happen on OEM paint jobs, but on repaints. When there is not enough dry time on base coats, or flash time on clear coats, this can occur.

Mixing chemicals. If you use materials that are not compatible with one another, this can happen as well. This is a primary reason why people suggest to purchase from the same family of products.

While there are probably more causes, your best solution is to sand it down and prepare the surface to finishing.

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It looks like rust that has begun to form under the paint or something failing with the clear coat. You may have to sand it down and refinish to fix this and keep it from getting worse.

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