Automotive paints, especially through a spray gun and at the right consistencies, will give you an amazing finish and you have tonnes of colours to choose from- but it is DAMN expensive. I work in an auto parts shop and so I can get it much cheaper and I mix it myself to whatever colour I want, and we have the equipment to spray it. As a result, my foam Reach armour has a beautiful finish- it does have something like 7 or more layers of paint however- and it has survived at least 20-30 troops without a full respray, only touch ups around the edges. That's not to say the paint hasn't cracked and buckled but it has held up surprisingly well and once you weather it a few times you don't notice that. I have friends who have used the dirt cheap paint at a couple bucks per can, they do a full respray every couple of events. Personally, since I can afford it and I know the longevity I can get out of it, I go with auto paint.
I have also used Rustoleum 2x paint on foam and that works pretty good too, nice coverage, nice depth of colour and you don't need many layers to look good. They don't last as long before requiring a respray, but those particular pieces of armour also aren't as heavily weathered so you notice the imperfections more. Personally I prefer acrylics to enamels. The cheap spray cans can do the job, but I prefer to spend a couple bucks more.
An important first step is to prime the surface properly, whatever that is and whatever method you use. Proper surface preparation can make a world of difference to how smooth and and clean the finish comes out and how well it adheres. Make sure you use an appropriate primer for what you are working on, eg when working with plastic best to use a plastic primer, when working with metals like aluminium you are gonna want etch primer. The rougher your surface in the first place, the 'flatter' your finish. If you want a nice smooth 'metal lookalike' finish then you want a nice smooth surface free of dust and contaminants. Also avoid painting when it is really damp and cold.... We just got an interesting matte effect on some of my armour as a result of said conditions