This is probably the time to spray exhaust and cordite stains. For some odd reason I always leave them until after the matt coat. If they are wrong, at that stage, they can't be removed.
When the wash is complete and dry then I spray on the matt coat, I prefer acrylic for this because it dries quicker. There is one more aspect to the weathering process- paint chipping. For this I use an artists silver pencil. It will not "take" on a glossy surface, so it has to be done after the matt coat. I do it after the model is complete, but before any antenna wire is attached. I will discuss it here, for continuity. It is very easy to over do, I'm sure some will think I'm guilty of that. I know you are supposed to have a picture of the aircraft you are modeling, but I very rarely manage that. I try and put the chips where logic would dictate, leading edge of wings (not all the way along, just in selected places) on rivets and fastenings, around hatches and removable panels and the odd panel line. On hatches and panel lines I try and have the chip emanating from an edge. I also do each side separately, without reference to the other, to help keep it "random". Panels that are removed more often such as engine and ammo box/gun panels receive a little more. Where ground crew walk and the pilot/crew get into their cockpits also receive a little more attention.