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Sometimes I just don't feel like priming some bits and just put on an extra layer or two of paint. Is it really that necessary to prime first? I know metal models its probably a must, but plastic or resin, I'm not so sure.

It's always a must. Without it, the paint isn't going to adhere to the plastic/resin very well. You may be able to get away with it, but it will wear away much faster than if it was painting on a primer.

In any case, paint is always slightly transparent, to let the colours come through. If you don't prime, then instead of a base tone coming through, either light white or muted black, your not going to get any sense of depth or illumination - just a faceless grey. Priming also gives you a really smooth coat to build upon.

Not priming plastic models is a lot like not chewing your food: yeah you can do it but not really advisable and things might go horribly wrong. You might choke.

Okay, I'm not really good with this comparisons.

Short answer; Yes, it is.

Priming is a necessary step for painting any object that doesn't have a natural tooth to it. Go ahead try and scratch off some paint with your finger nails... A primed model will resist this much more than an unprimed model. And hey that unprimed model you just scratched the paint of needed stripping and priming anyways, so scratching the paint was making a start on the process for you.

I find it impossible to paint to a decent standard without priming the model. Without it, you apply paint and it just thins out and the grey shows up, and with every brush stroke to cover that up, reveals more in another area. I also find priming allows you to actually see the detail of the model in order to paint in the first place.

Priming is essential, I never paint a single model that isn't primed. Period.

 Warpig1815 wrote:
In any case, paint is always slightly transparent, to let the colours come through. If you don't prime, then instead of a base tone coming through, either light white or muted black, your not going to get any sense of depth or illumination - just a faceless grey. Priming also gives you a really smooth coat to build upon.

that isnt true at all. especially with base paints being as good as they are these days.

but yes, you should prime your minis.

I've tried going straight to paint on a couple of bits I had forgotten to spray, just to save time. Never again. The adhesion was dreadful and there was no tooth to hold paint in place. I had to practically slather the parts in thick paint to get any buildup, as paint thinned normally wouldn't even leave the brush. The paint would always streak horribly - whether I could even get it to stay vaguely where I wanted felt like a matter of chance.

Plastics don't necessarily require a real primer, but they should always be "primed" (spray paint, gesso, primer - anything that sticks and has tooth).

I would be very careful of spray paints, though. because although they'll stick to your model most of them will not provide tooth. Especially semi-gloss or gloss ones.

Flat black spray paint is all I use. Resin, Metal or Plastic.

I also used to use a flat black spray paint. Worked fine. Nowadays I use a primer red and a primer grey spray. No noticeable difference except it's less dusty.

The problem with saying "just spray paint" though could lead to problems. I've learned this the hard way when searching for spray paint to use to prime models. I used a shade of blue I liked 9hey, no more basecoat!), then ended up with a surface you can hardly paint on.

Plastic is oily even if you don't notice it. This can interfere with paint sticking on the model once it is dry. Primer has a solvent component that lightly eats into the surface of the item being primed and allows it to stick better. The paint then sticks to the coating of primer.