sanding hull lines

320 grit sanding stick and 600 grit sand paper to start. 320 will knock down any ridges you might have (my upper hull was slightly wider  than the lower hull) quickly but leave some scored plastic behind. 600 grit in the opposite direction (I essentially sanded vertically with the 320, and then along the length of the hull with the 600) will take down most of the roughness fairly quickly, but if you want some help spread a thin layer of CA glue (AKA super glue... be sure to keep your fingers moving quickly so they don't have a chance to set on the model and stick) in the rough area to act as a thin filler.

A cross-hatch pattern with decreasing grits works well; if you always sand the same direction you'll never quite see when you've got all the previous grit's scratches out... this was a technique taught to me in airframe and powerplant school. We had to take surplus windows and return them to a crystal clear finish after the teacher scratched heck out of them with his car keys.

While using that rough of a grit might be scary to some, I find that overall it goes a LOT faster than trying to use an extremely fine grit that takes freakin' forever to do anything. Of course, it doesn't work as well if you have a hull with plating on it that you want to preserve, but generally this isn't a problem in 350th scale as plating shouldn't really be visible in that scale and most manufacturers are only too happy to not spend the time tooling up this extra detail.

Wet sanding works better than dry. It never hurts to prime it and see whats not looking good through paint.

I've had the best results with my few ships, which are in the 1/375 to 1/425 range, with making the hull completely smooth, and showing the plating very subtly with paint shades.

From what I read by the experts here, and my own half dozen models, the cast in/on hull plates on models are wildly inaccurate.

I assume we are speaking of steel hulled ships.