He was wiping only the sides of the metal part facing the outside of the gantry but not the top as he walked from one end to the other.
Could it be that he has already wiped the top prior to that?
Run your finger through the surface and you can feel a layer of dirt though you won't really see it with naked eyes. Anyway, nobody sees it or even bother about it.
Could it be that he intends to go another round to wipe the top later?
Looks unlikely since it doesn't make sense to do so, especially given his age.
Then did he choose to wipe the sides as people can see their reflections?
I don't think he is told specifically to wipe the sides only.