I know the original post is nine years old, but I found this post through a Google search about painting latex over spray paint (which I am encountering in a totally different situation.) I do have experience with painting over spray paint on a metal door 4 years ago (with great success) and we are once again doing this right now, so I thought I would share our technique/product in case someone else is in this situation. We used (and are once again using) Rustoleum Professional High Performance Enamel in a CAN (rather than spray paint.) It is an oil-based paint for metal. When we first used it 4 years ago, we were applying over faded spray paint. Spray paint is pretty durable for an exterior door but can fade terribly. We sanded any rough spots or scratches, and then used a spray primer over any bare metal. We waited a full 24 hours to do any more work. Then, we used a very fine nap small roller to apply the paint, using a small brush where needed (as it was a panelled door.) One coat of a deep red gave beautiful coverage over white primer. We waited a full 24 hours to rehang the door. The door today is as glossy and flawless as it was 4 years ago when we painted it, and it gets full sun, with temps well below freezing in the winter, and hot summers. This time, we're painting over another spray-painted door (should have done them all at once 4 years ago, but didn't.) We're also painting over a door which has never had anything but the factory-applied primer. In both cases, we are again sanding as needed, priming if needed, and applying the oil-based enamel with a fine nap roller and brush. This paint is durable, gives great coverage, and is beautiful.

Sanding would be ideal, but they're making nice improvements with bonding primers. You might try Ace Royal Sealing Primer, Ben Moore Fresh Start Superior Primer, Zinsser Bullseye 123 or Cover Stain, Sherwin Williams Adhesion, or Glidden Gripper (I believe all of the above say they'll adhere to glossy surfaces and oil or latex, but always double check the label before using a product). Paintergirl is right about the peeling of latex on heavily used items, though. I painted my bathroom vanity 7 months ago, and what started as a tiny chip on a shelf is trying to peel away into a bigger spot (I'll be going sanding & over it again with Ace Cabinet & Trim Paint, which is hard like oil but cleans up easily like latex). If your son is going to use that locker as hard as we did the ones in school, I'd definitely consider a protective top coat like poly (regardless of whether you use latex or spraypaint). Oh, and latex paint takes up to a month to fully cure, so if you go that route, you'll want to let it cure for a while (ideally a month, but a week would help) before putting it into service. HTH!

Most do, yes. You can drive by and inspect any house and see where the oil finish coats are by the amount of fade, usually as a dark color, like shutters? See if they make anything in acrylic that will give you a similar look. Acrylic doesn't fade near as bad but it absolutely must have a primer and anything with rust should be wire brushed and hit with rusty metal [red oxide] primer in addition to the whole-frame primer, which can be almost anything.

Well, planning a paint job 500 miles away with a ten day window has been tricky. I've narrowed my quandary down. (Will skip the 'rust conversion coating' since no one local I've talked to has a clue what that is nor carries it in stock. Did grab some wire grinder attachments for my drill, that should help with tackling the rust by hand. ;) Will have access to what looks to be a couple good products for exterior metal. Coronado's Coro-Bond or BM's Iron Clad. If anyone has experience with either product, I'd be interested in hearing about it. The only hesitation I have with the BM product is that it's a primer and topcoat in one product. Would definitely be a time saver, and am not sure exactly why it makes me uneasy, but it does. ;) Also it's oil-based, so going with white I'd be faced with yellowing. :/ So I might use the Coro-Bond primer and get a good latex top coat.