Before you do any painting, wipe off the model with some rubbing alcohol. This will remove any remaining sanding dust and oils left by your fingers from handling it during assembly. If you don't have any, run, walk, drive, take public transportation, whatever, to your local hobby shop and get some Tamiya Fine Gray Primer. Mask off the canopies and windows and shoot the primer. After that, take your finest grade sandpaper and give the primer coat a light sanding and then wipe it back down with rubbing alcohol to remove any dust. The primer coat will help the color coat adhere to the model, and give a neutral base, making it easier to apply the white for the underside. When it comes to masking, get your masking tape (Tamiya's is the best) and cut it into relatively thin strips. Use the strips to mask your cheat line (where the dark grey meets the white) and burnish the heck out of them. After you've masked the underbelly, go back and shoot a coat of white along the taped edge, this should seal the edge and prevent the dark gray from seeping under the tape. Shoot the topside color. When everything has had time to dry thoroughly, take a new #11 blade and GENTLY run it along the edge of the tape. This will give you a sharp edge and hopefully allow you to remove the tape with out lifting the edge.