Can you give us more info about the low spots and your work so far? I'm assuming these low spots are visible when you look down the length of the board at an angle? If the low spots are depressions that run across the short dimension, (at right angles to the edges of the board), they could be caused by the planer at the mill or by the drum sanders most mills use for production work. If that's the case, they never should have been shipped to you. Although we've seen a couple of recent posts about poorly finished boards from vendors, most vendors' boards don't require much sanding on your part...maybe once over with 150 grit, then once with 220, and a tack rag to get off the dust. Oak is a strong wood, but it's open-grained, and on new wood, you should use a grain filler. This is brushed on and wiped off and levels the grain for the finish coats. It won't fill in dips from sanding, though. In your case, if there are visible low spots that run parallel to the grain, i.e. along the long dimension, poor finish sanding may be the cause. Not being critical, but a lot of wood has been damaged by belt sanders left too long in one place, or by uneven pressure. Most bed boards from vendors require only an orbital sander or a vibrating sander to finish. Putting on more urethane will not fix the low spots. When you coat the board, you're adding the same amount to the entire surface, so the difference between the low spot and the rest of the surface remains constant. Using 320 grit is OK for light sanding between coats, but neither it nor 220 will take care of low spots now that you've got 9 coats of poly on the board. I know you don't want to hear this, but if you can still see dips in the surface, you may be looking at starting from scratch. The good news is that even 9 coats of poly isn't very thick, and a good chemical stripper with methylene chloride will get you back to bare wood pretty quick. Then use an orbital sander with 120 or 150 grit paper to get the surfaces completely smooth and level. Then sand with 220 before recoating. With polyurethane, you need to sand lightly between coats with the 320 grit to promote adhesion and you can put on as many coats as you want (although 3 should be enough).
