Hi Stu.

Like you, my modelling never included such things as primers back in the good old days of the early airfix kits (I don't think there were such things as primers back then!)
But (also back then) all the available modelling paints were enamel based and I think you could get away with a great finish straight onto the plastic (never even washed the parts in detergent like you really should!)
Today's paints though, at least the ones I'm using are increasingly acrylic and I think for that type of paint a primer or 'undercoat' is a much better bet. The acrylics will adhere to bare plastic but they fix onto the primer much better and give a stronger 'rub resistant' finish.
For a bright red model I would think probably a white primer, grey would slightly darken the red and a red oxide colour would do so even more.
I'm using the Vallejo Grey primer on most things apart from the metal colour airbrush range when I'm priming in black ('cos that's what the bottles tell you to use!)
Other than that, a matt white car primer in an aerosol will do the job (make sure its for acrylics and not cellulose though, the cellulose can do nasty things to some plastics!)

Best of luck.

Robin

I always wash the plastic in detergent to get rid of the oils from handling and I prime if I need anything other than a flat finish.I generally use the tamiya fine white primer for bright color final coats and I do sand down the primer to get relatively smooth finish to kake sure I get a nice glossy top coat.

Carl

I think white primer for red paint is a no no, any of you seen the way Austin 1100s went pink after a few years of sunlight (who remembers Austin 1100s)
I always use grey or red primer for red.

I only ever use real car paints in rattle cans so I'm a tad biased. White primer for red.
Grey or red oxide makes the red a lot darker.

Have a peep at my Gold Leaf Lotus 72. White primer everywhere before paints except black and silver, when I use grey primer.