You may actually want to scrape more off! 
Seriously, if a weapon had been used in the field for just a few months, it would likely be quite worn looking. If it's an AGM one, then the thick black paint is not very realistic. I used glasspaper to wear much of mine off, particularly on edges. Some people have painted theirs with a more silvery colour.
You've got nothing to ein, zwei, drei, vier
I sanded mine and then went over it with fire blacking on the metal bits, it looks much better now.
Cheers
Martin
"Mistakes in the initial deployment cannot be rectified" Helmuth von Moltke
Toys: AGM MP40, Cyma M1A1, TM M14/G43/SVT40, TM VSR/K98, SnS No. 4, ASG Sten, Ppsh.
Arnhem3,Gumrak,Campoleone
If you want it weathered, I would reccomend 'drybrushing' with some kind of silver paint (GW's Boltgun Metal is usually considered best). Drybrushing is putting paint on the brush, wiping most of it off on kitchen roll or the like, then gently brushing over the areas you want weathered
Nooooo!!!!!! Don't repaint it. There is a whole thread on here about how to weather your mp40 !
If you must repaint it then kryloan is the daddy of spray paint, but seriously, weathered is good. For further weathering I wouldn't go down the dry brush route with the AGM mp40 as it's mostly metal anyway. Dry brushing is gat for making plastic look like metal, but if you are starting with metal then just go at it with really fine grit sandpaper or wet and dry.
The paint on the upper receiver is tough as old boots.
