I would recommend a few ammendments to your surface prep. You can skip the initial wash if you want to. Start sanding with a low grit, I typically start with 120, then move on to 220 with a mouse sander. At that point you should have a much better surface to find imperfections that you can use spot putty to fill as needed, as well as areas that need more sanding. Continue sanding with 220 as necessary, then it's time to start wet sanding by hand, step up to 400. This is where you're going to have to develop sanding technique. Don't sand in circles, you want to go in as straight of lines as you can. Do the entire piece in one direction, then turn it 90 degrees and go over it again. Repeat that process with 800 grit. Most costume pieces will be ready for priming after 800. Do a thin coat, but make sure you get even and complete coverage. Sand that with 800 again. Recoat your primer.
Now, if you're applying paint, give it one last 800 grit wet sand and wash it with soapy water and rinse thoroughly. Once dry it's ready to paint. But if you're using a metallic paint, or rub n buff you're going to want to wet sand it with 1000, and maybe even 1200 grit, for the best surface possible.
A few extra notes on wet sanding. Wet sanding is the use of water to lubricate the sandpaper grit to avoid clogging, thereby prolonging the life of your sandpaper and allowing it to do it's job. I like to have a bucket of soapy water that I keep a sponge in and soak my sandpaper in for a few minutes before putting it to any surface. Use the sponge to wet the surface of your prop first, then start sanding with the sandpaper that has been soaking. Every few minutes as you notice the water drying, you'll want to splash your sandpaper in the water again to clean out the dust that's been building up, take a moment to wipe the piece with the sponge again too before you continue sanding.
I've now said sand and paper too many times, but I hope this is easy to understand and it helps.
