Lots of ways to do a wash, but here's mine. I thin the oil down so it's about like strong Coffee in color & consistency. I only apply an oil wash over a gloss acrylic top coat. Future is my favorite. Use a small brush, 3/0 or so, with a good point. load the brush with some paint & touch it to the recessed panel lines. The paint will flow for a short distance due to capillary action. Keep moving down the line till it's done & move on to the next. When done, carefully wipe off the excess paint on the model. I use a Q-tip moistened, not wet, with mineral spirits. If you remove paint from the panel line, just repeat the process till it stays. It's slow & laborious, but gives good results. Some like a sludge wash, but my experience with that technique has been mixed, so I'll let someone else describe it.
For an SEA camo, I'd use Burnt Umber for most of the wash, with Paynes Grey to accent control surface separations.
Regards, Rick
RICK
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