[Malcolm]

What type of fastener should I use? The fastener will be exposed. Should I use aluminum nails? It would be easier to use zip screws, but I'm worried about corrosion reaction between the fastener and the aluminum soffit.

I'm going to paint the entire soffit afterwords.

 

[PatChap]

If theyre going to be exposed, i would use stainless trim nails. If you use a pea shooter, they're pretty fool proof.
http://www.lowes.com/pd_76850-47120-SGY-TOOL1_0__?productId=3615428&Ntt= Sold everywhere you can buy the soffit usually.