Why does road salt reveal this bus's frame?
No. the best two explanations don't need any chemistry at all.

If you look closely at the panelling on a bus like this you will find that there is physical  evidence on the outer skin about where the structural frame is. The skin immediately over the frame tends to stick out a little. These physical features are probably enough to ensure that more dirt sticks to those parts of the skin. You can see a similar effect above warm radiators when small bumps on the wall above tend to pick up a disproportionate amount of the dust in the moving air caused by the convection currents from the warm radiator, becoming darker as a result.

Another possible contributor would be the fact that the vibrations in the panel skin of the bus will also be higher at the centre of each panel farthest away from the frame. This might contribute something as well.