Although WW1 aircraft were not spray-painted, one substance certainly was liberally sprayed about and that was oil! This is where things can go horribly wrong; having spent a fair amount of time painting the model it needs to be “dirtied up”, paying special attention to any white areas. An error now and it’s a re-painting job.
Ideally I’d have finished all the painting before I started on this stage but as I’m still waiting for the PC10 I’ve made a start where I can. Here’s the rudder after this second stage of weathering.
I apply the paint, which is very watered down, with a rag and wipe it in the direction of the airflow slowly building up the “dirt” to what I consider to be an appropriate amount.
The under surfaces of the tail plane really get the treatment, as will the under surfaces of the fuselage and the lower wings.
It is impossible, because of the lack of quality of photographs from the time, to be sure just how weathered / dirty these aircraft became in service but it is clear that the fields got quite muddy so I think it’s a fair bet that the aircraft themselves got equally dirty.