I am finishing an oak table. I lightly sanded the polyurethane after the first coat, as per the instructions. After the second coat of polyurethane dried, I see sanding/brush marks. If I don't sand again, will the third coat of polyurethane cover these sanding/brush marks?
I would sand this coat with a much finer grit of sandpaper than you used on the last coat. Then wipe it down with some alcohol before applying the next coat. This will help remove the deeper marks and the sanding dust. Also, use a foam roller or foam brush instead of a bristled one.