220 sandpaper. I have used steelwool 0000 to take down a gloss coat to semi-gloss. But you are trying to get out rough spots, impurities and get a flat level surface. Sandpaper is better, plus, some wood reacts to steelwool fibers. To be safe use sandpaper. Just get the surface level and don't sand thru to the stain if you are using any. You will load the paper up pretty fast with white residue from the urethane. Change paper often.
