Just ran across this thread, also new here! Though this was a good place for me to start!
Anyway, the aging you do to the cover depends on the look you're going for. Many helmets that spent time in country in Vietnam being worn often usually have fading, tears, stains and so forth. Graffiti often faded away and was either written over with something new, redrawn, of left alone entirely. It isn't uncommon to see multiple layers of graffiti on a helmet. And the key to good looking wear is red dirt. If you're going for the Vietnam wear look red dirt/mud was always all over the helmets and stained them pretty bad in alot of cases. But if you're going for a home front type helmet they weren't nearly as beat up. I had one with graffiti from a marine who was a radio man but never left the states during the war and he was ordered to cover the graffiti at one point and blacked it all out then drew more later.
And a side note though not completely relevant to the original question. The helmet and liners you choose will add authenticity to your helmet and plus even be historically accurate.