Hey everyone,
I have an old Cannondale CAAD4 (aluminum frame) that I’ve been riding for years without issues. Recently I noticed some paint bubbling on the rear triangle, mostly around the dropouts and chainstays. It’s been like this for a while, but I never really looked closely until now.
The spots feel hard to the touch, not soft or flaky. The paint is raised, and it looks like something is going on underneath, but there’s no cracking or visible frame damage yet. From what I’ve read, this might be some kind of filiform corrosion? Is that something common with old aluminum frames?
A few questions:
• Should I be worried structurally, or is this mostly cosmetic?
• Is it worth sanding down and touching up locally, or should I consider stripping and repainting the whole frame?
• Is there any way to stop it from spreading without a full refinish?
• Anyone with CAAD experience, how long can a frame in this condition keep going?
I’m planning to ride it until I get a new bike, but I don’t want to risk a catastrophic failure if this is something more serious.
by manceBre
9 Comments
Limited experience on this particular issue, but I think the move is to sand the damaged paint away and if you see any cracks in the aluminum, stop riding and replace asap. No cracks? keep an eye on it and ride on
Weird looking for sure. I would personally scrape the pain in a small spot to see what’s going on. I think aluminum will corrode rather than rust, could still be in big trouble if you’re able to treat it. Would probably require getting rid of paint as well. Yes, you should be worried!
Definitely looks like some corrosion forming under the paint.
Could be paint failure but could also be the frame holding corrosive water and it corroded and is bleeding through and causing corrosion under paint and blistering. Recommend removing some of the paint to investigate. If there is white powder under the paint it is corrosion and most likely corroding from the inside out
https://drivealuminum.org/wp-content/uploads/2022/05/Aluminum-Association_Corrosion-Types-Automotive-.pdf
My understanding of aluminium corrosion is that the aluminium oxidises & forms a protective layer on top of the metal thus protecting it from future corrosion.
Most likely galvanic corrosion. Where the tubes meet the drop out would be a joint. If they use different grades of aluminium they can still cause galvanic corrosion.
Sand it back, etch prime and repaint. You’ll at least slow it down.
It’s not really harmless. Just indicative of the paint/primer failing. The best solution IMO is to scrape away the bubbling paint and sand until you get to the raw aluminum. Maybe clearcoat over that area afterward.
I had this ALL over a retro caad frame that I was building up. My plan was to just scrape away the bubbling spots, but what I found underneath was a powder like material that I was worried about possibly being oxidized aluminum. Just to be safe, I stripped all the paint and redid the paint and clearcoat with spray cans. But I later realized/found out that it was just the primer, not aluminum. This looks exactly like that and is common on older Cannondales.
Sand, or scrape away carefully, check for cracks in the frame. Might just be moisture getting under the paint from dings and scratches. Best to make sure there isn’t a catastrophic failure.
Commenting because i also have one of these frames with the same thing happening.