If painting with a foam roller (used to me mohair, foam not as good) you have to use twenty quids worth of materials perfecting the mix Otherwise you get the orange peel effect and is hard to get an even pattern on curved body panels. Easier on a fire door in a council run infirmary. Secondly you have to be using the correct paint. Anything fast drying gives the paint less time to relax and even itself out before skinning. Same with acrylic paints, they dont relax because of the lack of oils. Any road up, so far you have the wrong paint and the wrong application method.
High volume low pressure spray guns are an all in one unit that runs off a hoover motor. Mostly used in the spray tan industry. Yes they are a little rough. Yes the spray mist is not exactly fine and no your not going to win a concourse prize at the local vicars feit. But. they give a better finish than brush or roller and you can get a decent one for thirty batts all in. Paint again is a big factor so your much better off getting a polyurethane vehicle enamel than anything cellulose based as this is slow drying and gives you half a chance of success. Even better are the matt and semi gloss vehicle enamels as they are much more forgiving on blemishes.