Most modelers know that when they feather the colors on model airplanes, the feathering they create is out of scale.  If you look at the camouflage paint on real WW2 planes you notice the line between colors are generally sharp... because they were usually painted with brushes.  Today, most planes are painted with sprayers, but even then the feathering on most models is out of scale.  If you have a one inch feather on the real plane, then on a 1/48th scale model, the feather should be 1/48th of an inch... it would be very narrow.  And on a 1/72nd scale model the feather should be even smaller.

But, when we paint the plane we know it "looks" better with a little feathering.  So how do we get the right amount?  I have found a nice little way of getting a very small feather when I air-brush.

As all modelers know, the closer a mask is to a surface, the less feathering there will be when we air-brush the camouflage on the surface.  So the goal is to hold the mask to a consistent, close distance from the surface of the plastic.

I use parafilm (a thin sheet of parafin wax), or Frisket Paper as a mask.  Cut the shape you want in the parafilm, then press a piece of fishing line into the parafilm very close to the edge on the side you will place against the surface of the plastic.  Then when you apply the parafilm to the plane, the fishing line will hold the edge of the parafilm a consistent distance from the surface of the plastic.  You can even use a tooth-pick to adjust how far the parafilm is from the surface of the plastic after you apply it to the surface.

I usually put the fishing line about 1/16th of an inch (~1mm) from the edge of the parafilm, so that when you apply it to the surface, the fishing line does a nice job of giving a very small feather to the paint.  This holds the mask above the surface a little less than the thickness of the fishing line.  So using different fishing line can vary the distance, and thusly vary the amount of feathering.  This makes an effect that is closer to being in scale than is usual.

You want to make sure the fishing line is far enough from the edge so that the paint does not reach the fishing line when air-brushing... a little experimenting will find just the right combination of fishing line size, distance from the edge, and painting technique to get the affect you want.