Hi Martin
Here we go in order:
1. For an incredible wood effect, I'm amazed by what the figure modellers do using oils... :-) I think they use an acrylic undercoat of tan colour, then Raw Umber applied lightly and streaked with a dry brush to create grain... it takes a while to dry, but the effect is awesome!! :-)
2. Yes always... you can airbrush it, or apply it by brush. This is much less hassle. See Holdfast's great Feature on his Bf110G - he explains the technique in detail. I also polish it when it's dry to get a high gloss ready for the decals.
3. You can use isopropyl alchohol, but Klear does tend to "gum up the works" so I usually give my airbrush a proper strip-down and clean it with cellulose thinners. (Hence the attraction of hand-brushing...!)
4.As Teacher says, make sure you wash off any decal solutions... then I give another coat, or two of Klear to even the decals out, and then a finishing sprayed coat of Micro Flat if appropriate.
Hope that makes sense...
All the best
Rowan