Is somewhere from a 300-600 grit sand paper good for wet sanding?
Yes, that is pretty much the range for wet sanding, depending on what stage your rocket is at.
For sanding primer, I use 320 grit DRY to get it smooth.
But than again, I build up with many coats of contrasting colors as a "guild" to fill the
spirals and "rats in a gunny sack" texture without any sanding in between primer coats.
That way I Can sand down and hit a certain "color", then check the filling and either continue sanding dry, or apply more primer and repeat.
After all that, and spirals are filled I'll do a "check sand" (usually dry with NON wet or dry paper) to make sure there are no missed spirals or paper tube popping up under the last sanding.
After all THAT, I'll add at least coat or 2 of primer, then I'll wet sand with 400 grit wet or dry sandpaper, pretty much as les has mentioned above using a bowl of water and sand/wipe dry/repeat and check again for missed spirals and high/low spots,
once the primer is complete and the body tube is covered, you can wet sand if you don't use too much water.
If I find a missed spiral, I'll prime over that area and sand smooth again.
As far as color sanding the paint goes, wet sanding shouldn't be a problem as long as it again aint too wet because the BTs are now pretty much sealed.
Hope that helps, that's just the way I do it.