There should never be a problem painting a further coat of enamel paint over a previous coat. In fact, it's normally better to apply two or perhaps three, thin coats, rather than one 'heavy' coat, and this is 'standard practice' among many, including professional painters, decorators and modellers etc.
Looking at the car photos, my first impression is that the paint has been been sprayed too heavily, and too close, possibly going over the entire model with more than one pass, resulting in too much paint in one go. When spraying, and particularly when using a spray can, it's always a good idea to build-up the paint coverage with multiple, light coats, allowing each coat to at least be 'touch dry' before applying the next coat.
The first coat may look uneven and patchy, but this will begin to develop into an even, smooth finish with the following coats, and once the entire area looks smooth and even, then it's done - this might take two, three or more coats, probably just two or three, depending on the colour of the surface being painted, and the colour and type of paint.
Thinking about the problem you had when brush painting, that sounds rather as if the pant hasn't been stirred thoroughly before use.
Regardless of the type of paint being used, for example, enamels or acrylics, they will always need stirring before use, and sometimes during use, if this is prolonged.
This is because the pigments, the materials which give the actual colour shade, are 'suspended' in what's called a 'carrier', which allows the paint to be applied and flow easily. With enamel paints, this is basically a light oil, and with acrylics it's an acrylic synthetic material or water-based material. When stored, the pigments 'settle out', partially separating from the 'carrier', and will settle in the bottom of the tin or jar.
Check this when you first open a paint jar or tin, and you should see that the surface of the paint, for some depth, is 'thinner', and maybe even a different colour, or even virtually clear liquid, whilst the bottom of the tin or jar will contain a thicker sediment, sometimes almost like tar, which is the pigment concentrated in just a small proportion of the 'carrier'.
Stir this 'mix', gently at first, until the entire container is equal in appearance and viscosity or 'thickness' - this could take anywhere between a couple of minutes, and ten or more minutes, depending on the type and colour of paint, and how long it's been sitting on the shelf.
For brush painting, have a look at the 'guide' I posted a few years ago - "Guide to brush painting" - in the modelling threads, I think under 'Painting questions'.
Hope this helps.