Alright, as example I take the front/rear arch
You have multiple colours and tones, which is a (very) good thing. But they dont blend smooth enough. Also, the arches dont look well shaped, it looks a bit wobbly. Thats because the different tones/colours dont blend the same into the side of the car. What you can do is to create a layer of the side of the car and make a new layer with the arch. But make the arches 'bigger' and then delete the part of the arch that you dont need with the eraser tool. The result will be that the whole arch blends better into the side of the car and all tones/colours blend the same.
Combine your brushing more with the pentool. Suggested that also to Nordic Man, he said it works better and make the brushing looks smoother indeed.
Hope you'll understand