Dirty landing boards, does plastic show it more

[charentejohn]

Just looking at the hives up close and saw the reduced entrance was getting really black.  This was, and is, reckoned to be propolis and stuff they bring in from where they are foraging.  Just wondered how many people have noticed this.

The bees arrived in may and on new plastic floors, they were shiny so I took the shine off with sandpaper.  After a few weeks I found black lines in the plastic radiating out like a sunburst pattern, obviously the directions of many take offs and landings.   More in the more popular areas.
I noticed this more today and wondered what it actually is.  Photos show clean in June then dirtier later and worse in the reduced bit.
I am guessing this is because of the plastic rather than wood ?  The stuff lodges in the grooves made by the sandpaper, looks very dirty just inside the entrance.

Another clue may be time of year as clean for a while in may then noticed it.  Now mid October and cold and wet but they are still busy middle of the day, foraging mainly Ivy at the moment.
I wouldn't disturb them now but could clean it with warm water and a cloth in winter if it is necessary ?  They will probably clean it themselves in spring, no worse than most solid floors I guess.   

[Michael Bush]

The bees often polish the landing board with propolis.

[charentejohn]

Thanks that makes sense and looks right from what I see inside.  The inner part of the landing are is very black, the outer bits scored with sandpaper have black in them.  I wonder if I should have left it smooth ? no worries though seems as ever they are sorting it to suit themselves. :)

[AR Beekeeper]

Wash with soap and a cleaning pad, then follow with a water/bleach solution.  It could be the beginning of mildew growth.