First off, a precaution: if you sand or file a helmet shell, you are removing parts of the shell and will automatically void the warranty.  DO NOT USE THE HELMET FOR ANYTHING OTHER THAN A DISPLAY MODEL!  Some helmet manufacturers claim that simply applying spray paint to a helmet can cause it to become brittle; obviously by sanding/filing a helmet shell you are making the shell weaker.  So do not use it for anything other than display purposes only, in order to properly protect yourself.  To avoid any confusion with SilverState55.com's shells, since they will only be used for display, NOCSAE certification molding on the rear of the shell is sanded off and all labels but the sizing label inside are removed.  Once this is done, it is recommended that you DO NOT SELL, GIVE, OR TRADE this helmet to anyone, to avoid any potential misuse of the helmet (and potential liability on your part).  There are plenty of wannabe rocket scientists out there who are all too willing to misuse a display helmet, especially if they've been loading up on "liquid courage" or "hero juice" (insert alcoholic beverage of choice).  Be sure to protect yourself, don't let these helmets get loose once you start to refurbish them according to these steps.

With that said, let's get busy!

Once the shell is stripped down, remove all labels, stickers, and decals.  Various methods can be used, but I usually employ an Xacto knife with a chisel blade to remove these.  For exterior decals, use a hair dryer to heat the decal and peel it off.  For residual adhesive, use either a Brillo or SOS pad to scrub off the adhesive, or steel wool with hand cleaner.  3M makes an excellent adhesive remover that won't damage most painted surfaces, available from most autobody supply shops.

Notice some things about this helmet: it has been painted at least once before (since it's a nice lacquer, it was probably done as a recertification procedure) and it uses both Velcro and plastic pins to mount the inner pads.  Riddell shells usually have various combinations of these, so be careful not to damage them unless you plan on replacing them.

I've also had to drill out (extract) various screws on facemask clips and chinstrap snaps that have become so rusted/corroded that they could not otherwise be removed.  If you need to do this, be careful and use the appropriate safety equipment.