A must for all paint jobs is sanding. You will typically start off with a lower grade sandpaper around the 150-200 grade range. Then go up to 320 to make it smoother. Any higher will be necc. for a shined finish. Now if you are going to be using any kind of paint on a beige mac, you should deff. use sandpaper first. I would imagine that using some spraypaint, e.g. Krylon, would not make the apperance look any different than using an airbrush. It is the surface you are working with. On other kinds of Mac's, like later generation ones, airbrush is typically preferred because the surface is harder to work with and will give a better shine to it. Someone can take off from here.
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MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop