Du to the vibration, small cracks in the bodywork revile them self's
Here you have two that I'm repairing
Rear fender:

Cleaned it as good as I could, and cut it in to remove the rust in the crack it self

Then weld it, carefully as its not that thick material

Grind the weld until its flat, the rest is for the painter

Here is another one, Front fender

Cut it to remove the rust

Welded it at the back

Grind the rest smooth, the rest is for the painter