So we hired a contractor to refinish our hardwood floors. Went to the house today to check the color of the stain and saw these obnoxious marks on the floor. They were not really visible on the sanded floor untill the stain was put down. They appear to be marks from a sander but I don't know. When I questioned the contractor he said it was from a previous refinishing, however we never saw these marks on the floor prior to this project. Is this possible? or did he church it up and is blaming it on prior work? The only difference between before and after was the prior floor color was lighter and this time we used choose minwax dark walnut. Can anyone offer any help?

https://www.dropbox.com/s/rmfhw2ptcjkoh ... 0.jpg?dl=0

Here is a folder with some extra pictures.

https://www.dropbox.com/sh/usroth2tg69v ... AEica?dl=0

These marks are from the latest sanding process. If the marks were from a previous sanding they would be lighter because the old finish was lighter. If there was any old finish on the floor from the last sanding, the floor would have been still sealed where the marks are now darker. All of the old finish must be removed where the stain has become darker in the machine marks from the latest sanding job. Apparently, your contractor has trouble with dark stains. Machine marks like the edger scratches will show up as darker cross grain swirl marks when stained with a dark stain where lighter stain colors or a natural finish will not show these marks. I believe that some of the marks are from the floor getting wet somehow, which opens the grain and allows the same color of stain to be much darker from being "water-popped".
A good sanding will remove previous surface defects from poorly sanded floors.
You might have the best work your contractor can perform. I would think twice whether you want them to come back to redo the sanding and staining, again.