The exterior doors are hurricane rated and aluminum frame. I prepped the doors and repainted the first set with grill paint. I was not happy with the finish and decided to repaint the remaining doors with appliance paint. The appliance paint left a much better finish and I now want to repaint the grill painted doors. Can I just apply the a appliance paint over the fresh grill paint or do I need to remove the grill paint first or prep the doors somehow?
You do need to have a primer for sure.
Just a tip from someone who paints 20+ metal doors a year. Use automotive spray paint. Cost a couple bucks more and you can pick it up at local parts store or online. The colors are better, then finish is better, and they make some great looking clear coats. Two coats of apple red auto paint + 2 coats of clear and I can turn a $20 door into a $500 door. Sean is right too after your first coat you should give it a sanding. No matter how well you think you cleaned it there will be dust that binds. (you can also take your door to an autobody shop and most will paint it for $100 - trick of the trade is to take in 5-6 at a time for around $250 if you use common colors)
Clean it well and then sand at 400 grit or rougher to give the surface some roughness so the new paint can adhere. Don't go too rouch or else the scratches will show through. You can go as low as 80 grit if you use a fill primer before painting. I'd say 220 to 400 if you don't plan on using a primer. Check the directions on the paint you plan to use - they should tell you what needs to be done. But don't be surprised if it doesn't look great in a few years - I doubt appliance paint has the UV inhibitors necessary for the pigments to stand up to sunlight.