Hey dude! Welcome. I would start off by getting yourself a dremel. You'll want to get most of the excess material off of your helmet, and armor with the dremel wheel. Afterward i would suggest using a sander to refine the edge to the place you actually want it to be at. That way, you don't risk cutting off too much material with your dremel (it's easier to control how much material you take off with the handheld sander).
Generally the rule for your armor is to leave 2mm-3mm of the return edge left. You can mark these measurements with a sharpie or something similar so you're aware of how far you can potentially sand it down to.
You can get a great dremel and handheld sander on Amazon.
The sander I bought:
[URL unfurl="true"]https://www.amazon.com/WEN-6307-Variable-Detailing-Sander/dp/B072Q2FTLY/ref=mp_s_a_1_14?dchild=1&keywords=handheld+belt+sander&qid=1622838659&sprefix=handheld+belt+&sr=8-14[/URL]
To prep your surface, I generally like to wash and scrub all of the parts with dish soap and let them soak a bit. Sometimes there are chemicals left behind from the casting process that can create a bad surface for your paints. Before you put any primer down, make sure you sand all of the surfaces. I recommend 400 grit sandpaper. This will ensure you will have good adhesion with your paints. If there were any imperfections in the cast (like bubbles for example) you can fill them with a spot putty, and sand it flush with the helmet surface.
