There seem to be some fairly widespread misconceptions about how modern hobby paints work. Maybe a little discussion is in order to clear it up. My own experience has generally been with PolyScale (my favorite), Testor's Acryl, Testor's enamel, Model Shipways acrylics, Humbrol enamels, Pactra enamels, and a few brands that are now defunct. I've fooled around a little with Tamiya and Gunze acrylics, but not enough to comment intelligently on them. Maybe other Forum members with other experiences will jump in.
I'm assuming that we're talking about name-brand paints sold specifically for use on plastic models. (Artist acrylics and oils do take some different application and preparation techniques.) Hobby paints from the major manufacturers are designed to stick to plastic. They do not require primer or anything else to make them stick.
When styrene plastic comes out of a mold, it sometimes has a thin coating of a substance called "mold release" on it. That material can inhibit the adhesion of the paint. It's a good idea to start any plastic modeling project by washing the parts, either in a substance sold for the purpose (e.g., "Plastic Prep") or diluted dishwashing detergent. I confess, though, that in 49 years of plastic modeling I frequently haven't bothered to do that, and have never observed any ill effects.
Occasionally one does run into a piece of plastic (in the products of small, cottage-industry manufacturers, for instance) that has a waxy finish to which paint has trouble sticking. But the styrene from the major kit manufacturers is generally excellent stuff. If the paint won't stick to it, something's wrong - either with that particular bottle of paint, or with the way the modeler's applying it. In either case, it's highly unlikely that a primer will help much.
The only real use for a primer on styrene plastic parts is to give them a uniform color and texture. Sometimes light-colored paints have trouble covering dark-colored plastic in one coat. If the model has had filler applied to it, the filler and the plastic may look different under the paint. And if one color of paint is trying to cover two contrasting colors of plastic, it may take an unacceptable number of coats to prevent the different colors from showing through. (On that Heller Victory that I was sent for review many years ago the hull was molded in yellow and the rudder, for some reason, was bright red.) In a case like that, a thin coat of grey primer would make it easier for the paint to do its job.
Modern hobby paints generally give excellent coverage and durability. Any paint applied to a non-porous surface can be rubbed off if it's subjected to sufficient brutalization, but if you're reasonably careful that shouldn't be a problem on a model.
It's worth noting that enamels and acrylics behave a little differently when they're drying. As it was explained to me by a scientist once, enamels dry from the inside out, whereas acrylics dry from the outside in. Most acrylics seem to dry faster than most enamels, but that's a little deceptive. Neither is actually dry (i.e., as dry as it's going to get) for several days - or even weeks. If acrylic paint tends to rub off, that's probably because it isn't thoroughly dry yet.
Some people have the notion that dried acrylic paint is vulnerable to water. Not true (unless you've done something very wrong in applying it). Dried acrylic paints are no more susceptable to water damage than latex house paints. The chemists (geniuses, in my opinion) long ago figured out how to make paint water-soluble when wet and waterproof when dry. (If they hadn't, imagine what the houses in your neighborhood would look like after a rain storm.) As an experiment, I once painted a 1/700 tugboat with Poly-S (the predecessor of PolyScale), let it dry for a couple of weeks, and submerged it in a jar of water. A year later the tug was still under water, and the paint showed no sign whatever of coming loose.
In any case, the paint shouldn't need to be "protected" by any sort of finish. Theoretically, I suppose, anything that increases the thickness of the finish will make it slightly more resistant to abrasion. But it's hard to imagine a scenario in which a model would be struck by something that would take off the clear finish without taking off the underlying paint as well.
Clear finish coats have two actual purposes: to change the degree of glossiness, and to make stuff stick on top of the paint. (Some modelers use pastel powder for weathering, for example. That stuff will rub off if it isn't held in place by a fixative.) One of the most common uses of clear finishes these days has to do with decals (which aren't common in sailing ship modeling). Decals stick much better to glossy surfaces than to flat ones, so the modeler sprays the area with a gloss finish. When the decals are dry, a coat of flat finish makes the whole surface, including the decals, look flat again. That "evening out" characteristic may also come in handy if you've used different brands of paint on the model, or for some other reason they have different amounts of gloss. (That can happen. Every color has a different chemical composition, and one "flat" color may be a little shinier than another - even if both came from the same manufacturer.) Another use of clear flat finish is to camouflage mistakes. When I'm working on a model's rigging I keep a bottle of PolyScale clear flat on the workbench all the time. Before I quit for the evening I take a careful look at the model under a bright light, and if I find any tiny shiny spots, where drops of glue show or I've inadvertently scratched the paint, I give each of them a drop of clear flat.
As I mentioned in an earlier post, I'm a big fan of PolyScale clear flat. I usually apply it with a brush; I defy anybody to find brush marks in it. Occasionally, on airplane models, I've airbrushed it, with completely satisfactory results. I've never known it to blush, or otherwise misbehave. I'm always nervous with spray cans; they have their uses, but they just don't offer the control that a brush or airbrush does. And clear sprays are hugely vulnerable to atmospheric changes. (I wonder if the humidity of Taiwan had something to do with Spelunko's problem.)
One of the huge virtues of modern hobby paints is that they cover with such thin coats. One reason why old ship models show so much unpainted wood and brass is that the paints of those days couldn't give thorough coverage without obscuring detail. Today we don't have that problem - unless we create it ourselves. Yes, if you insist on applying a couple of coats of primer, two or three coats of color, and a couple of coats of clear finish on top, the details of the underlying parts are likely to start disappearing. But there's no need to do that. Apply the final color directly to the plastic. Buy good brushes. Get some practice with your brush technique (or airbrush technique, if you insist); practice makes a good deal of difference. When you're done painting, take a good look at the finished product. If it has a uniform, even finish with no objectionable variation in gloss, leave it alone. It's finished. If you aren't happy with the degree or uniformity of the sheen, think about applying a clear finish - but be careful with it.
Too long as usual, but I hope that helps a little.