A short synopsis:
Most WWI crates were factory finished in gloss dopes and/or varnishes. Since the life of an airframe was measured in months, if not weeks, they really didn't live long enough to "weather". Here's what would have happened to the typical airframe:
The aircraft may have undergone a short factory test hop, was wiped down, disassembled and shipped to an air park, where it was reassembled. At this point, it was still glossy, but with a few chips around removable panels. When it arrived at its assigned unit it may have gone directly into combat, and had unit markings applied as operations permitted.
In service, the high gloss dope would begin to dull on the topsides, and dope and varnish would be scuffed, or at least worn thin, where the pilot scrambled up the side to get into the cockpit. Rotary engined craft would quickly start to grow staining where castor oil accumulated along the bottom of the frame, and the bottom of the airframe would be blackened by oily exhaust. In-line engined aircraft would begin to develop residue from oil leaks around the lower engine cowling. Depending on where the stack was mounted, there could be a bit of exhaust staining on the fuselage, wing or horizontal stabilizer. Fabric fuselage covering would begin to sag and even crack in the area of the the lower wing as the airframe flexed in flight and during landing. Unit markings may have been painted in thin paints that could allow the underlying colors to show through a bit, and could begin to fade or flake off in short order. Operation from dirt/grass fields would cause grass-stain discoloration (if nor outright damage) where the wheels kicked up debris onto the wing underside. Ground crews, of course, tried to keep the airframes free from accumulations of grime, but surfaces inevitably became stained. You can also add chipping around access panels and to the forward portions of struts and engine covers. Certain panels could show signed of having been "massaged" when the didn't fit quite right. Where the armament was close to the bodywork there may have been erosion of the finish, particularly forward of the gun muzzle (but probably very minimal gunpowder staining.)
Clear doped linen would show staining most readily, particularly anywhere the linen contacted an airframe component and crud could accumulate. Aircraft in the Middle East would probably show the most fading due to the harsh sun. Fabric damage was repaired with dope and whatever fabric was at hand. Some bullet holes were repaired with a small circle painted with the national insignia of the perpetrator. So, while a WWI plane never gets worn to the same state as some WWII birds, there's still plenty of room for a bit of dramatic detailing, IMHO.
HTH
Phil "Silverback" Schenfeld