I painted my MWS a couple of months ago. I separated the upper and lower and then taped off the gaps on the inside, closed the dust cover, taped off the trigger and the end of the charging handle to prevent any paint/spray getting in the inside of the gun, then put the two pieces back together. I too kept the inner barrel in and just taped off the muzzle device which worked. Just tape off anywhere you think it’ll get into the inner workings of the gun. Hope that helps!