Hey there buddy.



Starting at the very beginning, prep. What prep work do you do to clean and key the mags before painting? Usually there are years of muck and grit on the parts you're painting. It's always best to give everything a damn good wash before painting. Soap and water will work fine as long as you dry it thoroughly.



Then you will need to 'key' the surface of the plastic. Sandpaper is a good choice, anything above 100 grit will work as long as you're not after a super glossy finish (in which case you'll need to go for a really high grit). A green scotch pad will work too but I find a soft wire brush to be the easiest. It gets into all the nooks that you cant reach with sand paper. After you've keyed it, best give it another wash to get rid of any dust.



Adhesion promoter is a good call but you will also need a primer. It will provide you with a solid, even base to then apply your main colour to. That should prevent the variation in tone you can see in the picture. I generally let the primer cure totally before applying a top coat, several hours at least. You will need to factor the conditions in which the paint is curing. If it's hanging in a damp garage it will take longer than if it is in a warm workshop (I wish I had one of those). This is especially true if you intend to mask areas, it doesn't matter what tape you use, it will lift the paint if it hasn't cured properly.



If you're using that horrid beige masking tape then you will never get a good edge no matter how you try. The green 'Frog Tape' in B&Q is pretty good. Better still, head to a hobby shop and get some modelling masking tape (usually blue and really thin).



When it comes to paint, it's quite difficult to judge what will give a good effect, especially if you mix brands as you are. A bit of experimentation will hopefully lead you down the right path but generally it's easier to stick with one brand of paint. It can be very limiting but it's a good place to start.



As for getting an even coat and getting the paint into all the nooks and crannies, technique plays a big part in that. How you move the rattle can and the angle you hold it at is crucial for even coverage but again, there is a tip that will help you out. Warm your rattle cans before you start spraying. I fill a bucket with warm water and leave my cans in it for a good 10-20 mins before I start and then give them a damn good shake. You'll find that the paint will come out in a much finer and more even mist, giving you more even coverage and it'll help the cans go further.



tl;dr You're doing well. Keep at it and eventually you'll find a technique that works for you.