Originally Posted by fishacura
Great feedback and thank you for your assistance. Is there such a thing as "clear semi gloss paint"? I didn't want to change the color....or for that matter a latex based poly???
There is absolutely nothing wrong with applying one or several coats of an oil poly over a flat latex paint. There will be no issues of incompatibility - they will work fine with each other. You are right that the oil poly will yellow some, over time, but over black it really won't matter or be that noticeable. If you are concerned about yellowing, there are latex (acrylic) polys in a semi-gloss finish that are pretty durable - and would work fine.
As long as the black paint is clean, and in good sound condition, applying a coat of oil poly is a great way to provide further protection to your lockers. The poly will NOT wear off any faster around the handles than would an "enamel" or "semi-gloss" paint - matter-o-fact, the oil poly will provide greater protection than either around the handles, knobs, whatever.
The biggest concern I'd have with the oil poly is applying it in a basement, in a laundry room - but that'd only be if your basement is not dry, or if you already have a mildew problem due to high humidity - if so, mildew can grow on an oil (alkyd) finish easier than on a latex (acrylic) finish. If that's not an issue, stay with what you've got, it'll be a well protected project.