Just posted a photo! Do you think the same contractor could fix the issue? Do good contractors have occasional issues like this?

Seems to me the floor would be repairable only with a rotating disc sander, like a U Sand machine. Reason, a drum sander may, unless the operator is highly experienced, just make the waves worse.

The disc sander will take longer, but can flatten the high parts of the waves. 1,200 sq. ft. may well take three or four days, as there will need to be at least four grits used(80, 100, 120, and 150).

After sanding and before applying finish, dampen an area of the floor with the solvent used in the finish to be applied---water for water based and paint thinner for oil based. Just damp enough to show any problem areas, since the liquid will shine like the finish for a short time.

Then lightly sand that area, since the liquid can raise wood fibers in the flooring.

Thanks so much for the comments. FYI it's water based poly if that makes a difference. Seems like sanding is the culperate though.

Based on the image, the contractor has a mechanical issue with their drum sander. The drum can be out of shape, worn shaft, bearings or wheels out of round. The waves can be fixed, but floors will need complete rework. They need to spend a lot of time on the finish sanding. If, they do not have rotary sanders like a Trio or power drive. They can use a multi-disc driver plate. They will need to start from 80 and work their way up to 120. Very important not to skip grits. Going beyond 120 is not recommended, because finishes may not penetrate and hold. Although it can be done, but that's another subject.

Just for clarity, since I am not a flooring finishing expert. My advice on sanding to 150 grit is based on years of finishing wood products like cabinetry/shelving/frames/etc. to get a smooth even surface for the finishes.

It does look like a rush job with a bad machine. Probably a resand as you have to sand out the waves. Using an orbital machine like handy mac suggested would result in a very even, flat surface, Good luck.

Hi all, the contractor is redoing (there is another issue) but is saying the waves are from a springy floor and there's nothing they can do to fix. I don't remember them there before but I'm not 100% sure.

Does this sound possible? Or would an orbital sander etc fix the issue regardless of the potential springiness of the floors?

From your photo, the waves aren't the only problem. Filler should be present in the spaces between the boards. I think this was a rush job and it was poorly done. I really don't believe that the problem with the waves is because your floors are springy. I think it's their drum sander. I am wondering how many people they've told the same story to about those waves... What sort of contractor is the person who did your floor and did you check to make sure he has a license? I would not let them do any further work on the floor, especially since they are telling you they can't fix it and also because there's more than one problem with their work. I agree that you need someone with a trio, or one of the other (similar) machines. If you let them sand more on your floor, when they are telling you they can't fix it, you are just letting them sand wood off of your floors, reducing the lifespan of your wood, for no reason (because you'll end up with the same results, and it'll need to be done again). You will definitely end up with waves again when they use the same drum sander they used the first time.

"but is saying the waves are from a springy floor and there's nothing they can do to fix."

What?

Although, it is possible to get waves from a springy floor. The refinisher needs to know what type of equipment and process to use, in order to properly finish a sprung floor. A rotary sander is needed. If, they do not own one. They will need to rent one.

Poor scheduling was done in order to meet a move-in deadline.

Simple sanding error here... sometimes a machine issue...sometimes a loose floor can cause waves to appear as shown in your picture. Regardless, sanding error.

Sorry to post on an old thread but I can't figure out how to make a new post. We are having a similar issue. Floor guys have refinished our perfect floors to make them darker and royally messed them up. I have one room satin finish, one room and stairs gloss, and raining is satin. I've had them back 4 times and they stil mess up. Now that the gloss is on the majority of the floor seems like we are stuck with it but now you can REALLY see the waves in the floor. Any thoughts on how to handle? Just pay and find someone else? Tell them to redo the whole thing or I won't pay the rest that I owe (half). They seem to think they are doing a great job?