There are various methods of doing "wash" for different purposes. I will try explain this old methods of using artist oil paints for plastic modelling. "Washing" wud means cover the model with heavily diluted oil paint which I did for my TF MP-03 Starscream.
First of all, paint your model with acrylic based paints & cover with a coat of clear matt finish. Once done, I use "Raw Umber" artist oil paints diluted with lotsa enamel thinners. This will give a filthy brown colour suitable for most aircrafts. (Reason to use enamel thinner so that it will not "melt or dissolve" away your acrylic base paint) Apply the mixtures with a pointed brush along the engraved panel lines. Wait for few minutes to dry. Then use a clean cloth, moist with bit of enamel thinners to wipe over the surfaces. As you are doing so, the cloth will leave a thin layer of the oil onto the surface, as well as build-up of paint on panel lines & any raised details for that "weathered look". This should be very subtle & not too heavy effects. Lastly, spray a coat of clear finish (either matt or gloss) to protect the surface.
Some modelling magazines (such as fine scale modellers & Verlinden Way)) will explain this technique in great details if you find them. There may be other new methods now of doing panel lines, perhaps our forum guys in model kits dept can tell us more, i wud love to learn them too.