That hardboard is decent stuff but it can be a bear to paint. The shiny side is tough to get an even finish and the "rough side" is a sponge. Still a good product for the money.
The concrete pattern is there from the pour and curing. I'm guessing they covered it with plastic to protect from rain or debris. It then probably rained which caused the plastic to dip and sag in certain areas. The marks and lines are where the plastic was in contact with the concrete while...