400 is way too aggresive in my opinion. You can use it, and you can remove the scratches from it, but 400 cuts so fast that it makes it very easy to sand through. My last two builds I started at either 1500 or 2000. It went pretty quickly. If you want to get it level fast and then work up, I would start around 800 or 1000. The idea is to use the finest paper possible so you have less chance of deeper scratches. If you can start at 2000 it is great, because those are very easy to remove, but if you need to do some average leveling, 800 or 1000 will save a lot of time, and is still easy enough to remove scratches.