Am overhauling an early 70s road bike that has some corrosion in the usual places. However, there are some "snakes under the paint" on the underside of the downtube with no obvious entry or exit points. My question is should this be a cause for concern or just a minor cosmetic issue? I'm addressing rust that has surfaced with judicious use of a trusty Dremel tool and sandpaper so I could go after the biggest offending snakes if they will pose a major problem in the future.
Advice sought and appreciated!

I'm a builder and painter and when I've seen this before, it's usually because the tube is rusting from the inside out. So most likely it is not just a cosmetic issue but a serious cancer. That doesn't mean it will die soon and if you ride it, it will break catastrophically and you will die too. What happens is that there is not uniform rusting so the whole tube is thin. Rather after awhile small holes will appear (and maybe they have already) along the worm areas. However it only makes sense to start planing now for its replacement and keep a close eye and any developments. There are options to fix it but not cheaply.

Sometimes when it is painted they haven't used a primer 1st as a base so the paint is not sticking tight to the steel and water gets in there to start the damage. This is more often true with powder coating.

Very informative post! I sometimes paint my own frames but luckily have always started with a good canvas both external and internal. I have seen "snakes" but wasn't sure what it meant. Thanks.

Any chance of a picture? I googled "snakes under the paint" and I got a lot of pictures of snakes. Also tried "rust worms" and got lots of pictures of worms.

I have a frame with a few thread like marks on it and I was wondering if this is the same thing.

As Doug points out, it's not good. As a frame builder they see more than their share and often have to break the bad news to folks that don't want to hear it.

I have seen the worms find a weak spot that remains undetected until it was really bad. It can go inward and not seem bad, then the pinholes show up.

There's usually more than you think once you start looking for it, way more.

They can be very hard to see, some frames have them that can't be seen at all till the paint is off, then it looks like etching. You would be amazed how bad it can be.








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Here's some more.

Cyclops Frame with Some Rust Issues--When a Full Restoration is Not Intended

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Search "rust worms under bicycle paint" or some such.

Filiform corrosion aka "filamentary corrosion" or "underfilm corrosion".

I politely beg to differ with the dire prognosis suggested previously, as I've generally found filiform corrosion to be readily checkable with an acid bath, I prefer phosphoric. though I suppose oxalic would work. In my experience filiform has been superficial. I'm not saying it can't erupt from within the tube, but I haven't seen that often. Mostly it's sweat spawned.

Thanks Merziac. The frame I have has a few of those, though not as extensive as in your picture.

Here is a photo of "worms" on the bottom bracket of the same bike posted by [MENTION=425004]merziac[/MENTION] before paint removal.



In this case the rust was only under the film as described by [MENTION=452523]machinist42[/MENTION] above. As you can see by the photos posted by merziac some of the heaviest rust was at the top of the seat lug, an area where it was easy to see that they had not propagated from the inside out. I see these fairly often in the coastal environment where I live.
Brent

Absolutely correct in many cases, tx for the correct terminology.

Again as Doug alluded to it can most certainly be a scourge and kiss of death albeit maybe a slow one. Caught in time, not that big of a deal, left unchecked it can screw up a lot of paint which I would think a good idea to avoid.

Jim Merz treated his frames by complete contaminate removal and zinc chromate? primer before Imron paint was applied by Virginia for the infamous PDX rain. He is adamant that frames be stripped, treated and repainted when this is found, Between him, Mr. Fattic and many others, I'll go with the Master framebuilder's on this.

Also as you said it can be readily checkable, that's provided one is cognizant of it which many are not and that is what I think we are advocating here as two hadn't seen it before.

I also always like to learn all I can about new to me problems which always includes worst case scenario so it can be walked back from as necessary.

"'Plan for the worst, hope for the best"

Thx to all for this really valuable information. While working on the frame this afternoon, I tackled two of the offending "filamentary spots" and sanded the areas clean - each was roughly half the size of a dime. After sanding carefully, all that I encountered was clean bright 531 metal which was a relief at the time. There are some other filamentary spots on the underside of the downtube that can be tackled as well.
I'll take some photos of the "before and after treatments" for these spots and trust that you folks can steer me the right direction.
During the process of purchasing this bike, I learned that it had most likely had been ridden in the 1970s and possibly early 80s and then it ended up in the basement until last fall. The owner passed away in 2006 and his children were uncertain what to do with the bike. Although they were cyclists, a French road bike from 1971 was beyond their comfort level. I don't think that the bike was cleaned up before it was placed in storage as I'm finding that the corrosion on the frame is where road grime and sweat would have accumulated and over time, attract moisture and attack the paint. It is rare to find a large, vintage frame in my size, 64cm, so when I find one of these bikes, it becomes a mission to restore it to it's former glory and ride it in the CA Eroica event.
More info to follow tomorrow when there is better light for photos. Thx again to everyone on this thread!

Definitely agree with the latter assertions about the filiform being surface. On a car, it is usually from within but I have yet to see a bike frame come from within. Usually it is the result of a bad primer adhesion or a pinhole nick in the paint protection that allows the moisture or corosion process to wick along under sub par finish.

The situations I was referring to looks just like the yellow frame Merziac posted. Where the rusting-from-the-inside-out is most common is the very underside of top and down tubes and chain stays. If a person doesn't clean their bikes often the problem can go unnoticed. Sweat induced rust is usually found on the top of the top tube - especially around top mounted brake cable guides. I've done many repaints where that was the only issue. In these cases because the rust is starting on the outside early damage is visible and not as likely to be as destructive before corrective measures are taken.

Where inside rust creates holes is most common is when lighter heat treated steels (like Tange Prestige) began to be used. I don't remember seeing this nearly so often on heavier Reynolds or Columbus tubes from the 80's and before. Partly this is due to older frames having thicker walls. However different steels rust at different rates. One of my framebuilding class students had his shop flooded with hurricane Sandy so his inventory of various brands of tubes were immersed in a salty sewage brine. Some of some of those tubes rusted a lot and others not at all. Of course what kind of oily protection played a part too.

I made myself a fancy lugged frame back in the 80's. It was for a contest that Bicycling magazine did in conjunction with Shimano to market their Sante brand of components so I kind of went all out doing extra stuff. For the seat post binder I used the concept of a Cinelli 1R stem that had a push plate to secure the seat post so the seat lug did not have a visible binder bolt and the whole lug could be carved. While the concept worked okay to hold the seat post it also let moisture in the crack between the lug and post allowing water into the top tube. Eventually because I rode the bike a lot and sometimes in the rain, rust with small pin holes developed on the very underside of the top tube. When I repainted the frame to save it I used a kind of stuff that turns rust into primer (inside the top tube). After that I used a combination of silver and body putty to patch the holes and it still is working fine today. I remember rushing like crazy to get the bicycle together so it could meet the contest deadline and I didn't spray the insides with rust preventative.

I agree here with the above, any rust can be a problem, but the speed of collapse if the coating is very variable.
The start is often at a thin film (as in paint film) condition that starts adjacent to a sharp turn of shape, lug point, cable guide, etc.
I agree that when addressed in a reasonable time it is of no big problem.
Every powdercoated frame I have bought has had this rust issue to correct. All of them.

I did have a DeRosa that was a replacement for a failed frame, the original looked good, most likely the problem started with an incomplete or skipped rinsing step in the chrome plating process. pinholes from the inside out within 24 months of purchase. Stored indoors, no rain riding, southern california... De Rosa replaced it no questions asked.

My contributions - the technical term is "filiform corrosion" and is informally known as "worm tracks". You can find these terms in the technical literature. Snakes under the paint seems just as good as worm tracks.

The times I've seen it, the cause was traced to chlorine ions, caused by failure to properly clean a plated part.

Rust never sleeps. My bike has some under the paint. I'm too lazy and too stupid to do anything about it.

Snakes are gross.

My experience is that they are surface inhabitants.

P1010140, on Flickr

P1010167, on Flickr

P1010141, on Flickr

I am sure they will buff right out!


P1010183, on Flickr


P1010442, on Flickr
both sides are drive sides on a tandem.

Took 3 quick photos this morning to further show the story. Last photo shows what a little sanding revealed.
SJX426, I think that my frame is showing "surface inhabitants" similar to what your tandem experienced.

After a slight sanding.


The questionable area.