If y u do not "gut" the laptop you are running the risk of getting paint into problem areas. I happen to take laptops apart for a living... oddly enough so they can be painted where I work, lol. It is really not THAT bad, but it is a trying experience if you have never done it before. If you have a rather mainstream unit you may be able to find a guide online with step by step instructions. For instance, ifixit.com is an excellent resource for taking about Apple products.
As far as a finish is concerned, when you paint the unit you woudl have to apply a "clear coat" to accomplish this. Doing so woudl also make your paint job more durable. Although, overall I have yet to witness a spray can paint job that acually looked good in person.