Where has this gunk come from?

[Pgd]

Was just cleaning my bike for the first time in a few weeks (yeah I know) and saw this trail of black gunk along the down-tube. It seems to be something like plastic or rubber -- it will flake off with force, but not easily, and is resistant to soap & water. I've had the bike for some years and not changed anything (or noticed anything changing) recently. Any ideas?

 

[fossyant]

Recently laid tarmac - they use a sealant on the sides of repairs - maybe you went over that ?

 

[Drago]

A bit of white spirit or brake cleaner on a rag will shift that.

 

[ianrauk]

I had the same a couple of years back. As Fossy has said, its tarmac sealant.

 

[Pgd]

Aha, thanks all! Mystery solved. Will get out the white spirit.