This may sound dumb but what can be done about the brass deflector getting marked up??? I cleaned most of it off but there is still faint marks there?
Just wanted the gun to look as good as possible.
If its your fighting rifle then I would not be so concerned about “fit and finish”. Besides the marks add character and when I clean my rifles the brass marks always disappear.
It’s more of a “fun” gun for the range. Do the marks on the deflector detract from the value of the gun if I were to resell it???
Nothing. A worn Brass Deflector is a sign of propper ownership of said weapon. You should get that Idea of selling it out of your head. Take this and call me in the morning, as a Doctor would say. If you really want to get rid of the blemishes, I recomend Flitz Metal Polish. I intentionaly mark my guns up so that I don’t sell them. I used to buy and sell, and wish that I had most of them back. Trust me, selling a good firearm is akin to loosing a good friend. And if your upper is used but in perfect shape, minus the few blemishes, plenty of folks like myself would gladly buy it from you. I know, pretty rifles are nice, but unless you plan on photographing them for Playboy, it’s pointless to worry about cosmetics. Now, go feed your Rifle, as you have undoubtably hurt its feelings by thinking such things.
To answer the question, I have heard of people putting electrical tape or the fuzzy part of velcro on the deflector.
Touche, that is a very wise idea. I must admit that.
attach a small plate with an adhesive or tape over it… but the correct answer is to not get rid of a gun.
What, it isn’t as fun if it has some marks on it? 
Worrying about stuff like that will only affect resale value if you’re trying to rip off fools when you re-sell it. It’s not like we’re talking about something that is collectible here.
Coming from the FAL world- those are “beauty marks” and nothing to be ashamed of - just a sign that she’s no safe queen.
I spent 15 years as a USAF Small Arms Markmanship and Training Unit (SAMTU) instructor…we were so used to all of our daily use M16s that we trusted and knew functioned 100% that whenever we got new rifles in, we thought they looked weird…all smooth and shiny.
We wouldn’t really trust them until they looked as bad as all the good ones.
Use a small chunk of black electrical tape as mentioned above . It will stop the dings . Smooth it over the hump and you cannot even tell its on. I tape all of mine and people tease me, but all of my weapons look great .
This! 
Scars build character.
Guns need character.