I've got an option to get this road bike which has a thumbsized dent in the side of the downtube of unknown provenance. The frame doesn't appear bent (string through the dropouts and around the head tube) but given it's an aluminum frame, I thought I'd ask an opinion of the hive mind. I don't have a lot of experience with aluminum frames.

Fill the dent in with JB Weld, sand it smooth, and find an amusing decal to cover up the spot. I would be more concerned the frame is bent, that looks suspiciously like some sort of vehicle impact. If so, cars pack a ton of momentum and can very easily bend a frame in the blink of an eye.

What is it? Sone Alu frames i.e. my CAAD 12 are notoriously thin walled.

what does the owner say.

The frame is a Miele (https://en.wikipedia.org/wiki/Miele_(bicycle)).

The owner "knows nothing". I was thinking vehicle impact or attempted lock pry. I'll have a closer look at the frame for potential damage elsewhere. It's a got a decent grouppo so I may even just get it for parts.

-cam

Good idea wish I'd thought of that when I filled a dent on one of my bikes. Ended up using a blob of Sugru and put some reflective tape over it. Seemed to work pretty good but time will tell I guess.

If everything else is okay, I wouldn't let it bother me.

If you're going to cover it with an amusing sticker, there's no need to even bother with filling it with Sugru/JBWeld/Bondo/whatever; just cover it with the amusing sticker. Keep an eye on it for cracks, and if/when you ever decide to repaint, then go through the bother of filling it with whatever and filing it smooth.