Jedi Dade is right. It's often a "judgement call" depending on the kit, how the parts are assembled, how you want the finished kit to look, and so on. Sometimes it's better or easier to paint before assembly, sometimes it's better or easier after. There often is no right or wrong way.
That said, if you paint prior to assembly it is recommended to remove the paint from the two surfaces that are to be bonded so that you're cementing/gluing bare styrene to bare styrene, bare resin to bare resin, bare vinyl to bare vinyl, and so on (this is especially true of chrome plated styrene parts), and touch up your paint work after the cement/glue has cured completely.
Thanks for the info! So lightly sand the parts where the contact glue/cement are.. I plan on painting most parts before putting it together with this kit. I can't imagine getting my brush in and for example painting the controll panel or seats. Three main color's to the get two grey and one black.
As with the earlier replies, "it depends" is always true.
I do lots of model ships, so there are tons of parts that are just too dang small or fragile to paint once on the model.
Another good method is to build and paint sub-assemblies, then assemble fully painted sub assemblies. This helps for things like cockpits, since like you said, it is hard to get in there once its in the model. You can fully build the cockpit and detail it prior to putting it in by sticking it to a piece of plastic or cardboard with some tape of glue(very small amount) so it doesnt move on you.