Washes

  Ok thanks! 

No, by light  coat I mean needle driven in to give little flow from brush, and a transparent/translucent coat.  Color of underside depends on what effect you are creating. For oil I'd use a dark or medium brown, thinned a lot and applied from areas oil likely to leak.  Not so much dust and dirt for carrier planes, so go light on that, unless ground based, as at Guadacanal.

  Thanks guys! Don, for the white underside of a Navy plane, what color would you use, and I'm assuming by light coat you mean a thinned wash sprayed on? 

I always do my weathering with a combination of airbrushing and washes. I use the airbrush for light colors, with very thin transparent coats.  I use the washes for some darker stains, and for panel line enhancement and such.  Dark exhaust stains I find best done with an airbrush for better control.  But I like a brown wash for oil leaks as the wash kind of flows like oil does. 

Incidently, some aircraft pumped a lot of oil out the exhaust, and I do those with the airbrush too, airbrushing both the brown and a black/gray.

I did my TBD Devastator in a dark dirt using Flory's wash. that was blue gray/white. It looked great. Using oils I would suggest a raw umber, mix with gray wash, so it looks more brown than black. For fading just add white or light gray to the base color and airbrush it across the model very lightly.