Have a look around your local area and see if there are any small finishing shop businesses that can phosphate it for you?

Buy a very, very zinc-rich primer and put down a few good coats, then polish the primer - if you're above about 85% zinc in the finished layer, the paint will polish like pure metal but it's an intensive job.

Otherwise, be prepared that if you clear-coat a steel frame, it will begin to corrode under the clear coat, and sooner than if you'd got primer and paint between the steel and the gloss.