Chronology of Weathering
By this I mean when did each "weathering event" occur in relation to the other ones. Did the vehicle get muddy before the crew spilled petrol on it or after, was it dusty before driving through a bog? A history of the vehicles weathering should be worked out beforehand and the weathering effects applied in this order. In general it is best to apply the effects as I have listed them in this article. Shadows, faded paint, field applied secondary camouflage and winter white wash are actually part of the painting process so are obviously the first effects, in that order, to be modelled. Long term effects such as worn paint and old heavy dust should be next. Topical effects including rust and fuel spills are next. Finally mud, snow and new dust should be applied last.
It is worth mentioning that some of the materials used for later effects may damaged a previous effect. For instance, a water based wash used to simulate dust will destroy the secondary camouflage effect created with artist's tube water colours. To prevent this from occurring it is best to use dissimilar based paints, in the case cited it would be better to use enamel based washes instead.
Below are all the weathering effects that I can think of which can be simulated on a AFV. For good results, patience and practice are important for good execution. I keep and old junked model around to practice on (an old Tamiya Matilda, which now has all kinds of mud, snow and even white wash all over it)!
An often overlooked aspect in depicting weathering is the daily wear and tear the vehicles are subjected to. Everything from a small dent in a fender to it being completely torn off can add considerably to a vehicle's looking used. The key to realistic looking battle damage is to, again, simulate Nature. This means do not heat the parts up first and melt them into shape. Instead it better to thin out the parts to scale thickness first, and then bend them with tweezers. After all this is how it happens in the field. The parts which are usually subjected to damage are the thin sheet metal parts such as fenders and storage bins, and frame storage brackets.
Thinning these parts down is not difficult, however, it is quite tedious. I tape sand paper to a piece of glass with double sided sticky tape and make small quot;handlesquot; from masking tape to hold the part while sanding. This is much the same method that vacu-form aeroplane model builders use. Work slowly and check often to see if the surface is being reduced evenly; the best way to do this is to hold the part up to a strong light and see if the part is consistently translucent - if not, concentrate on the thicker areas. The finished product should be almost paper thin. For parts which are not flat, hand sanding will have to be used. Sometimes, however, it is best to completely replace the part with a new scratchbuilt item. I prefer to use 0.0050" plastic stock, but sheet brass can be used too.
Now that the part is thin enough it can be damaged. Do this logically: for dings and dents, lay the part down on a semi-hard surface (a hard back book is good) and push the part with a hard blunt tool; for bends, simply bend the part with tweezers. Sometime I chew (yes, with my teeth!) on the part a bit - looks great! Under no circumstances subject the parts to open flame or a hot-knife -- the part will look melted and not bent. This is just a general outline for battle damage, the details are best discovered through experimentation.