What kinda metal for tailgate skin?

[Senor_paco]

Guys, i was looking at metal for skinning my tailgate, and would like an opinion on good metal. Now this isn't a Sir Michaels sking, I'm doing it myself. But there are os many choices of metal, galvanized, galvannealed, the list goes on. I figure I'll buy a sheet and trim out what I need. I have some square tube that I'll be clamping to help form the metal to the tailgate, and will mig weld the sides, and can use a torch to help it lay flat if need be. I can drill holes in the sheet every 6" or so, and use short panheads or tack weld, grinding all smooth later.

any advice on how to proceed? all is welcomed. I plan on using 18 gauge, since I've been playing w/16, and it's a lot harder than 18.