Am working through weathering a Sopwith Camel. I’ve looked at dozens of photos and they show dirty birds, but are indistinct because of age. There are a few films in color of either original or rotary engined replicas: the owner of one said his camel burned 13 pints of castor oil per hour - indeed the oil loss was 100% if the plane exhausted its fuel. There’s a constant plume coming out from underneath the plane. I’d guess the color is a kind of oily brown - not black. (That makes things more interesting because there certainly appears to be darker soot around the top of the cowl in addition. And of course there’s dust and grass stains.) Right now I’m thinking of something like a light brown with some varnish in it sprayed thinly over the lower part of the aircraft directly behind the engine (this includes the lower parts of the sides). Maybe some green in it. But films lie like bandits. Anyone work with anything like this stuff. The oil would be fresh and then ejected immediately as it’s used - presumably darkened in the process. Would this stuff maybe be a little green? Dark brown, maybe even a very dark grey? I could use any of these colors based on the films I’ve seen.
The oil is clear and the engines worked on daily, however the fabric was pretty much left alone allowing the propwash to kick up dirt onto it. the soot was caused by burned petro from the exhaust and not the oil. So the motors were relatively clean even as the rest of the bird needed a real big “Biz Bag.”
Castor oil is (or so it says in the advertizing copy) “a colorless to very pale yellow liquid with mild or no odor or taste” UNTIL it is allowed to polymerize at high temperature, then it is a witch’s brew that resembles something between caramel and varnish. That’s why it is only suitable for a total-loss system such as a WWI rotary engine. It is necessary to drain the engine before it cools off, or you’ve got a NASTY job cleaning it out.
Varnished carmel sounds about right judging from the films. The film of a Camel and Spitfire flying together is pretty famous ( http://www.youtube.com/watch?v=j6PnKUEFX8g ) but I’d never seen it and it is an eye opener. You can see the castor oil constantly flowing down the side of the aircraft like exhaust from a oil-burning truck. And there’s a very evident large stain in the area right behind the engine. So it looks like you have three distinct processes that should be weathered somehow: the castor oil, the dark soot from the engine that collected behind the cowl and the dirt/dust/grass stains from the field that would have been across the bottom although much fainter than the castor oil splotch. I can certainly understand why good biplane modelers might chose “delivery day” condition. A really well made example is so pretty it would be a shame to mess it up in the name of history, and every plane would have had one delivery day. But my kits aren’t destined for any competition so we might as well give weathering a go - good history if nothing else as long as it’s done right.