Well as I've said this isn't that smooth yet just primed over 120 right now. But to answer your question.
I start with a filler file, 40grit, 80grit, 120grit and so on so forth. I also use files, need files jigsaw and hacksaw blades you get the idea. The helmet is the smoothest part right now being done down to 240grit. Which will then be sanded with 320, 500 and 600 wet and if it need primer between then i'll start at 500 again and then wet.