Jerry,
Zink chromate is an older bright yellow type primer which adheres to metallic surfaces pretty well.There are the newer green type primers which are better in my opinion but,I guess that's another discussion.I would recommend,that the surface be free of any dirt and oils and since it is steel,it's fairly easy to scuff up a little without destroying the integrity of the steel.If you are going to spray it on,you will thin it with toluene.Contrary to what a lot of people think,you can brush it on with good success as long as you thin it to brushing consistency with xylene which will give it a better wet-edge retention.
Joe Cygan
www.cyganfamily.com
Cozy MKIV #1022