Whether a garage door is metal, wood or any other newer material, an exterior primer paint made of latex will usually work well. If you are unsure, ask the sales representative when you are at your local paint or hardware store if the latex paint that you are considering is the right one for your type of garage door. Usually, it will say on the paint can what surfaces are applicable to use it on.
Make sure you that you thoroughly clean the garage door with soap and water or an all-purpose-type cleaner to get all the grime and dust off of it before you begin to apply the paint. Rinse it with a hose. Then, wipe it down so that it is completely dry.
It is easy to apply with a wider paintbrush, roller or even a sprayer if you are skilled in using one. It is always a good idea to lay multiple pieces of flattened cardboard on the driveway or ground in front of the garage door to protect it from the excess spray or drippings from a paint brush. It is a good idea to test the strength of the stream from a paint sprayer on one of the pieces of cardboard before you begin to spray the door so you can adjust it if it is needed.
You should a paint both the outside and inside of the garage door for continuity in the look of the door. Let it dry for up to 24 hours and then apply another coat if necessary.