Your AU-1 looks very good with the decals added. You might try mixing up some chalk powder, (using and earth shade), with some water. Try not to get it too thick, as you will want it thin and “soupy”. Brush it on and let it dry. Come back after it has dried and try misting some clean water on top of the plane, and let gravity do the work for you. I have used this method in the past on some of my armor builds. It just might work for you here. If not, simply wipe it off and you can start fresh……. as the chalk is not permanent until you seal it with some sort of clear coating.
One more thing about using chalk, is that once it is covered with a clear, the effects are not as pronounced as they were beforehand. You can always come back and add more later if it’s not how you like it the first time around.
Just food for thought. Hope it helps.
Spiros’ idea with the Q-tip and a paint brush is a good idea too.