I actually used 220 grit on the under surface of my slats.
I LOVE the way it turned out. In my practice with all the different paints, I was using 80 grit -> Primer -> 220 grit (for quickness, I was testing alot of combinations! Ha!) and I really like how the surface looked after only 220, so I figured 400 would be EVEN better.
Here are my slats. Again, I love the way it came out with just 220.