Paint touch up

[08montana]

As many of you have had problems with, my rear hatch on my Expedition has bubbled and peeled. I ordered some touch up paint with the correct color code to paint over the ugly raw aluminium spots. I got a sand-able primer, color paint and a clear overcoat in an aerosol form, however I'm not sure what the best way to go about prepping for and applying the paint. Any ideas on what kind of sanding or such I should do?

I know that I have to eventually have to repaint the whole hatch, this is more of a temporary fix.