Look into PPG's DAU75 acrylic urethane clear. I have had very good luck in the past using that on a bare aluminum valve cover on a Honda 4 cyl engine, given the proper cleaning and pretreatment with the recommended PPG chemicals. I have not tried it on bare steel.
I'll give you a link to the tech data sheet for reference. I have been unable to find a way to direct link to the pdf file on the new PPG site. Instead, the link should get you a dialog box to either download or save the file to disk. Clicking on download will show the data sheet for DAU75, assuming you have the adobe acrobat reader software on your computer. The clear must be catalized with DXR80 urethane hardener.
Here's the link: DAU75