Many many years ago some British modeller (who else!) developed the idea that not only human beings but also airplanes get older and show signs of use. But, he added at once, don't overdo it.
The idea stuck. Lesser so his advice. Therefore explanations why the model shown is so dirty (appearently having spent its whole life in a coal mine) are legion. Airplanes are greasy and sometimes dirty indeed, but not so much some "experts" make us believe. Mostly it's grossly overdone without any thought for the possibility of such stains and bare metal.
Did anyone ever see sand or stones on an aircraft carrier? So why is there bare metal on all leading edges of a carrier based F4U Corsair model to be seen? - Or, believe it or not, on a Mosquito!
Therefore the question is not "how to make it?" but W H Y.
Naturally bare metal shows only where metal is. Besides the "wooden wonder" (Mosquito) there were some Russian WW II planes with wooden wings and/or rear fuselages and wooden parts on many others (floorboards in cockpits, aerial masts and so on). Not to speak of fabric covering.
If sand is responsible for bare leading edges there has to be sand. And it has to be loose.
Upthrown sand removes paint not at once but gradually. At first the paint gets matt/e. As more paint is removed the primer showns through. And only when this is removed too, bare metal is visible. A thin line at first spreading with time and always (!) with soft edges as lesser stressed areas are just in the beginning of wear.
Signs of wear appear when and where wear occurs. If a mechanic has dirty shoes the walkway on the inner side of the wings gets scratched. Constant scratching can make bare metal visible. To produce such stains it needs time! On the other side: Where nobody walks with dirty shoes no wear occurs. Plainly spoken: It's complete nonsense to paint silver edges around all panels on the outer wing (or rear fuselage or horizontal stabilisers).
As wear from dust or sand produces soft edges sharp edged silver spots indicate damage. This results in a dent. Possible but not everywhere und not on the complete leading edge (got into a shower of stones?).
Dust in the air, sunrays, salt water, oil and dirty fingers leave traces on paint. It's irregularly bleached or darkened after some time. And that is the magic word! - T I M E. - It takes time to bleach paint (in the arctic it needs very long!). And if the airplane doesn't live long enough because of enemy or friendly gunfire, it will rest on the bottom of the ocean with some holes in it but with immaculate paint. Not to forget the innumerable variants of some type. Still flyable planes were replaced with newer/better ones (the former serving in training units in the future). And they usually arrived in immaculate state. If not in use and stored well the airplane remains in fine condition. If in use and maintained regularly, the plane will also look fine. If not maintained it will not fly. If fataly hit it won't fly either. If there are many adversaries or luck runs out chances of survival are low. So German and Japanese airplanes didn't grow gray hair in 1944-1945.
Another point: Not all paints detoriate the same way. Some are prone to fading, some not at all. Glossy paint is much more resistant to environmental influence than a matt/e one, and "matt/e" doesn't mean sandpaper like. Most so called matt paints were of slight lustre ("egg shell sheen"). If bleaching of paint occurs it depends on the ingredients of the individual paint to which colour it is fading (see page P, paint).
One point completely forgotten by certain painting artists: Airplanes are regularly serviced. Mechanics are proud of their aircraft and try hard to see it in pristine condition. If hard pressed the exterior of an aircraft could suffer, but never would he (or she) allow a rusty rudder hinge or other things impairing airworthyness.
According to a German wartime mechanic aircraft in the early fourties were washed (with fuel!) when time permitted and never were there any paint chips on leading edges and so on to be seen. The latter point was verified by another for the later part of the war (although in 1944 there wasn't enough fuel for use as cleansing agent). And, most important, according to the latter mechanic in 1945 maintenance was confined to refilling of fuel, oil and armament because either the plane was shot down or replaced by another variant.
According to a widespread legend Japanese airplanes were notorious for their bad and therefore chipping paint. - It's a legend, no more! Anybody telling this nonsense either didn't check photos or is totally unaware of the difference between Japanese Army and Navy aircraft. Admittedly when US troops inspected some remote bases they found some not so good looking wreckages, but are they representative? Japanese paint was as good as any as can be seen on wreckages found in the jungle many years later. When in 1944/1945 aircraft went down as fast as they left the factories, the lifetime of the aircraft was shorter than that of the paint. Even if it had been bad.
For ("bad") Japanese paint see also page J
Streaks behind engine exhausts also are time related for in regular use they don't appear at once. Whether they are black or not is another question. Normally they are light in colour and whether they leave traces depends on a lot of factors, not to forget the airstream.
The same goes for gunsmoke. A narrow streak behind a protuding gun is physically impossible.
For exhaust (and gun) stains see page E.
In summary: A lot of knowledge of the actual airplane, its service time, location of service and much skill is needed when trying to age a model. Overdone ageing is worse than none.
Patches of lighter/darker paint areas are reproduced best this way:
Apply a thin coat of the desired paint
Spray blotches in appropriate areas in very much lighter/darker paint
Overspray lightly with the original paint
The plus of this way: You can't exaggerate. If done too hamfisted the blotches will disappear.
Oil stains are best reproduced by wetting the approriate area with thinners first: Then apply a dot of slightly thinned glossy (!) paint and spread it with a brush so that the deepest colour is in front. As oil usually seeps through panel edges, mask the next in front to get a clear separation of paint and oil stain. Most combustion engines spilled used motor oil, usually on the underside of the fuselage or wing, so in this case - and only here! - one hasn't to be too timid.
Fuel stains are created the same way, but with very thin glossy paint (add a little drop of light brown to thinners) not spread by brush. On the fuselage simply let it run downwards, on wings blow it backwards.
(for example see F4U Corsair model)
Hydraulic oil ist often coloured (red or blue), used motor oil is brown. New oil is translucent and only motor oil gets darker by use. So for modelling purpose see for an according paint that remains somewhat translucent when applied. Never use pure black and never spray it by airbrush.
Exhaust stains are reproduced best with crayons. As exhaust stains seldom are of uniform colour, the type related colours are applied in a row and blended with a cotton bud. Mistakes can be removed with a moist tissue. A matt/e surface is preferable (mask roughly before applying gloss varnish). Airbrushing is not advisable because the paint feathers the wrong way (getting broader to the rear). At least if the exhaust tubes are fuselage mounted. See page E, exhaust.
(example for applied exhaust stains: A-1H model)
Also called propeller. Device to generate thrust by moving air backwards. Superseeded by jet propulsion.
In model form due to production limitations difficult to reproduce, especially in smaller scales. Mostly they are too thick and/or too rounded.
Making an airscrew is easier than one might expect: See page. "making of an airscrew".
Excluding recessed antennas airplanes shows blade-, whip- and pole antennas. On models they have one in common: They break easily. Especially such made from stretched sprue.
Blade antennas are best made from flexible plastic (PVC), cut and filed to shape and inserted in an appropriate slit. Very thin ones I cut and file from a razor blade. For whip- and/or pole antennas take a hair from a broom (preferalbly one of natural material) and insert into a tiny hole. For the bent ones seen on the undersides of F4U-4 and -5 thin copper wire does best. For manufacturing the complex array of German wartime antennas there seems to be no other way than soldering. Please notice: The vertical parts were held in clamps on the side of the horizontal parts, each with three different diameters (use thin tubes, to be seen at least in scales 1/48 upwards).
For fixing antennas in the fuselage or wing I use "plastic paste"(see there)