Wash it thoroughly in dish soap/water, rinse, dry well, apply primer in thin coat.

I think most people skip the priming step. Depending on the scale your modeling too many coats of paint will cover up the small details. Many times you will add dullcote on top of your finished model too. 

The only time I think you would need to prime is for painting light colors such as yellow. Modern paints have no trouble sticking to styrene so like Alan said wash, rinse and let dry (cleans the mold release agents off) and if you do need to prime (not for adhesion but for color) make it a very thin coat.

Primer can aid in reducing edge pulling - thin color on sharp edges (outside corners of detail). Especially noticeable with poor hiding colors. The low surface tension of plastic can cause paint to pull back from an outside corner just after application (during flow time). Primers have much shorter flow times than paint. The short flow time results in the primer setting in place before it can pull back. The surface tension of dried primer is much higher than that of plastic so paint does not pull back as severely over primer as it would over plastic. 

  If the kit has several different color plastic parts, I would primer the model.  I usually primer kits to make sure there are no glue marks or rough edges - it makes it much easier to 'see' imperfections with a thin coat pr either grey or red primer.  Also, it gets rid of paint showing differently on various different colored plastics.

Jim

Thanks guys for the replies. I think I will prime it.

I seem to recall some time back that someone made the statement that most model paint "primers" were really not true primers, but simply regular paint with a primer color.

Does anybody know the validity of that statement?

Floquil/Polly S have a color called "Primer" it is not actually a primer though,

I use Tamiya Grey Primer. Good Stuff!!

OKGraeme

Check the labeling and the information posted on the manufacturer's website.  If the description says that it should be used to prepare the surface of the model, then it's really primer.  Otherwise, it's just another color.

Thanks,

Damin.

Hey,

I used Tamiya grey primer (fine) as well. It is good and one of the best they say. I made only 2 or 3 pieces of plastic kits and I always applied a paint directly on surface. But this is not what I want, so I was thinking maybe I should use a primer. I primed the kit yesterday and today (probably) I will proceed with painting.

Thanks guys and cheers,

In the UK, a common source of primer is the auto retailer shop, using car primer in an aerosol. Cheap and come in grey, red or white. A can goes a long way.

George