Wet sanding urethane

I would use finer paper/block that that. And I've never used the soap mixture. I've been painting cars for about 13 years and have never had any problems with in your case 3000-4000 grit and water. But be real careful. I'm just making a guess. It will be real hard to make judgment with out being right there, so I would try the 4000 and if it is not cutting it than go up to 3000 and ect. You might even want to get even finer. It really all depends on how bad the orange peel is. I've been known to use all the way as small as 6000. Or even just masking paper. You would probably not be able to use paper on that kind of paint though. I hope someone else will chime in here. It is alway's nice to get some other opinions when your dealing with this kind of thing.