Simulating the look of wood

I do it the other way around, in fact, the last time was a hobbycraft camel in, I think, 1/32 scale. awful kit, needs lots of work. Anyway, I use humbrol enamels.
1/ start with the base colour, a mid brown,say.
2/ Once that has almost dried, dry brush a lighter brown in the direction of the grain, just thin wobbly lines really.
3/ rub down lightly when dry, micromesh if you have it.
4/ Varnish in gloss or semigloss as required.
5/ practice makes perfect, try the above methods on a sheet of plastic card first, when you're happy, you'll know how to do it, & you'll still have a useable sheet of plastic card, not a ruined model!

Well for starters I wouldn't airbrush wood. This is how I paint it on rifle stocks in 120mm.
1 start with a sand or brown colour, Anywhere between desert yellow and dark earth, depending on what you what the finished product to look like. The lighter you start the more the 'grain will stand out.
2 Using brown oil paint (a mixture of Raw umber and Burnt Sienna is good) apply this pretty thinly, brushing in the direction you want the woof grain to go.
3 Leave this for a while to start to dry, how long depends on the type of paint. Once it has thickened up a little, use an older, stiff, flat brush and gently run this over the oil paint in the direction of the grain. The idea is to remove some of the paint, leaving lighter areas and simulating the wood.

In larger scales the slight texture of oils is handy, but you'll want to get it pretty smooth in 1/48. You will need to leave this to dry for about 1 week if you use oils.