Had ours lined with 1/8 inch plastic for maybe 12 yrs now.  Replaced it once because it gets too scratched up.  Did it mostly so cows don't sort.  And yes, it helps eliminate that scum build up.  Biggest drawback is plastic expands and contracts with temp.  Ours is held down with wood strips along the edges to allow it to move.  Still gets wavy in hot weather.  In a perfect world I would epoxy the cement.  Can you powerwash yours and epoxy it?