what kind of brush to use.
Best way to get a flawless finish would be to remove the door, mask off any glass etc and spray the door.
Make sure the door is preped good. Its not about the brush but more about the paint! The paint should be an outdoor paint suitable for metal applications! Good luck!
I had the same problem, but I also had some rust on the outside. If there is rust, you should do a sanding to get rid of the rust. If no rust, wipe it down with mineral spirits to get rid of any grime, and then apply a coat of Kilz oil based primer. This will get rid of any discoloring or stains, and also prime the door for the final coat. You can use any exterior paint, latex or oil. Be advised that using oil based paints in humid areas like Florida will encourage mold growth. I went with a good exterior latex and it looks great. You can get Kilz and paint at Home Depot.
I just did this successfully. First, take the door off the hinges and lay on saw horses, out of direct sunlight, if you are painting outside. Remove the door knob. Wash the door thoroughly with soap and water. Rinse allow to dry. Mask off the hinges and tape newspaper to windows, if any. Spray a light coat of primer on the door. I wanted to spray my door with Rustolium spray paint, but the color I wanted was not available in spray paint, so I got paint meant for metal surfaces and I rolled it on with a small foam roller for an even brush stroke less finish. I rolled two coats of paint on the door, allowing the paint to thoroughly dry between coats. You cannot even tell the door was not painted by a professional.
