I think the best reference we currently have for what a WWI aircraft might look
like is to look at the ones currently flying.
Granted, operational museums like Rhinebeck, Old Warden, and Owls Head go to great
lengths to keep their charges looking good but some things are inescapable. The
oil stains from the engine being most notable. The aircraft operated by these
concerns are pampered but they are also much more than the few to several months
old that most arcraft at the front enjoyed as an average lifespan. They do not
see much mud which, as noted, was quickly removed, but they do suffer long dusty
months of storage between seasons.
As long as they have not just been recovered/restored they give a good
impression of an old bird living the good life. Of course in 1918 the Sopwiths
and Aviatiks were hardly old birds. They were the pride and joy of the airmen
who flew them and except for unavoidable "events" they were probably treated
pretty well then also.
Thayer Syme
San Francisco