The main reason why paints can't mix is because of the solvent.
Let's say you want to spray acryllic-on-acryllic. Let's say both types use thinner as the solvent. If the bottom layer flashed off but not fully dried (i.e. the thinner hasn't fully evaporated), and you spray on the next layer, the thinner from the bottom layer will try to escape to the top and end up creating bubbles or uneven paint surface.
The other possibility is the thinner from the upper layer seeping through to the bottom layer, or 'attacking' the bottom layer. Another scenario is when the thinner at the bottom layer needs to evaporate, and the thinner at the top is attacking the lower surface. You end up with the layers not sticking together, uneven surface, paint creep and other such symptoms.
Hence, the need to spray really thin layers at a time, first and foremost. A primer is just a layer for the desired paint to stick on. Choosing a primer also depends on what surface the primer can adhere to. Generally, once you get a primer that can stick on a surface, your job should be easy from there. If you have a problem of your paint not sticking on the primer, then you have to experiment with introducing the paint onto the primer surface during flashing, right before or right after flash-off. Let the surfaces bond together due to the solvent action.
