Using gaussian blurred layers for the rivets and panel lines is a useful weathering shortcut, but its not the whole answer. This sort of thing looks too uniform by itself. You need variation, some bits of an aircraft get much dirtier and worn than others.
Firstly, with your existing rivet/panel line dirt layers, try making them a little more variable. Select an eraser tool about 10 pixels wide and with low density and medium opacity. Use this to make angled semi random sweeps all over each layer. You can erase them more in the cleaner areas of the plane.
I like to create an overall dirt wash too. This is done using a wide dirt coloured airbrush tool set to very low density (1-2% typically). I spray this over the whole painted area of the airframe, concentrating on the dirtiest areas (wing walkways, leading edges of airframe, undercarriage etc.) Once done, this is gaussian blurred by a pixel or two. Then the layer gets faded down so it just barely visible. I usually do this layer twice, but with a different colour and using motion rather than gaussian blurring on the second layer (in the direction of airflow on each panel).
A dirt streak layer would add a lot to the skin. Use a 5-6 pixel medium density airbrush tool to create a variety of different streaks. Each streak should get narrower and fainter to the rear. Like the streaks you have on the bottom of the fuselage, but they need to fade a bit more to the edges and most of them should be much fainter.
Once you have 10 or so streaks of differing size, density or colour, you can copy and paste them around the plane. Place them behind panel lines and hatches and anywhere fluids would leak (cowling hatches) and in the direction of the airflow. Turbulent air created by imperfections on the surface allows dirt to stain the surface. You can partially erase or colour change some of the streaks to add more variation.
Once done, each weathering layer should be faded down so it just barely visible. It can be frustrating to make a load of work almost vanish, but you don't want any one effect to leap out at you. The whole thing should blend in to a grubby looking whole.
One thing I do on my skins is create two strengths of panel lines. One layer is all the heavy lines; engine and gun hatches, undercarriage doors, control surface edges etc. The other is all the lighter lines, airframe panel joins etc. Try copying your panel line layer, then erasing all the less important lines on the copy.
Some more paint chipping on the edges of the cowl and gun hatches would look good.
The exhaust streak need to fade at the edges a bit more, maybe a gaussian blur?
That's all I can think of for now, hope its been of help.
