http://www.misterart.com/crafts/decorative-painting/faux-finishing/triangle-coatings-sophisticated-finishes-metallic-surfacers.html

Also considering using this product instead and simply painting the sheet, then doing a rub through in different areas with some sand paper or steel wool.

Thanks for the response, JDS. I'm a little worried about the use of acid so I'll continue actively searching for better methods. The paint above looks promising!

Galvanizing is a zinc plating that doesn't take paint well unless it has been well cleaned especially if it has been allowed to weather. Acrylic paints are best.

I wonder if I could spray the sheet down with some vinegar and leave for a few hours, then rinse well and apply the paint? Maybe that would help remove some of the zinc plating, or at least "rough it up" so that the paint adheres to the surface easier.. Or simply buy the painted metal sheets vs the galvanized?

htwo82, please be careful with that stuff. The fumes from that stuff are something serious. I had a co-worker that spilled a bucket of it cleaning a concrete porch and passed out almost immediately. Somehow he was able to get up and make it to the yard for fresh air but he doesn't remember any of it. He said he remembers spilling the stuff, catching a big whiff of it and then waking up in his yard. Said his throat burned badly for a while after that.