How do you paint worn weathered wood?

Depends on the scale, and whether you want to see grain of wood. In small scale, where you want to merely represent the color, I often apply a flat gray first, then airbrush a quite thin layer of color over it. It can also work the other way around- do the color first and then airbrush a flat gray over it in a VERY thin coat.

If it is a large enough scale that you want to see grain texture, I sometimes draw a dremel tool wire brush along the surface that gives a sort of grain texture before I paint.  One then must not apply primer and color  too thick , lest you fill in the texture.

Kentucky Colonel

I need to paint a large part pf a plastic model as worn weathered wood.

I have Tamiya, MM, Vallejo and some Polly Scale paints all of them are acryic paints. I also have Mig and AK weathering sets so I'll be using them. Vallejo has an "old wood" paint but it doesn't look right.

What I'm trying to do is make the wood look like it's been outside for a couple years without being painted. I'm going to do washes and dry brushing but what color paint do I need to use?? Should it be all gray or a wood paint with gray dry brushing?

Any ideas would be a big help..

You might want to do some searching around a model railroad forum ( like http://cs.trains.com/TRCCS/forums/default.aspx?GroupID=8 ), old boxcars used to be made from wood so weathering wood something that is often talked about over there.

What kind of wood?  Different woods vary greatly in color & how they weather.  You got yellows, reds, greys...some can even go green as the sun & rain & snow take their toll.