Here's the finished project, tried the filter technique for the first time, came out better on underside. Also tried to enhance the fabric covered area of plane with buff. Not sure if it's necessary or accurate but it makes the area stand out in a subdued way.
Hey I hope these pics help put you at ease, because as you can see the RAF, in various schemes, utilized very hard and semi-hard demarcation lines. So if you can't nail the blurred lines, hard edge is perfectly fine and probable for your Spit.