I see waves on my non-tensioned screen, how do I fix them?

Our QC standard for matte white non-tensioned screens are as follows:

Turn the lights off and project an image
Stand at the diagonal size viewing distance. For example, 120” diagonal screen = 10 ft.
At this distance you should not see any waves or lines. Having the lights does not determine our QC standard. Lights that are on cause reflections/shadows which can easily be seen on the screen’s surface.
Non-tensioned screens can never be as flat as tab-tensioned screens. But with the lights off they can be sufficiently flat to produce a great image.

Below are some examples of how a non-tensioned matte white screen with the lights on reveals waves vs. lights off an no distracting waves on the image.