first of all - your stuka looks great!
This truly is a heavy worn Panzer IV. Great weathering on this one. Really fine! How did You do this chipping?
I used a sponge/small brush technique.
Light colour of base camo first - sponge moistened in paint (I remove quite decent amount on towel paper, almost like with drybrushing ) to apply random spots of paint chipping in desired area. I also add some scratches and marks with small brush to improve chipping results and variety in some areas.
Next I repeat same technique but with dark brown colour to achieve rusty look. Also - sponge and later corrections with brush.
The important thing I learned is that it's good to hit surface at random angles at same spot with sponge, otherwise chipping will look unnaturally duplicated.