Spent hours doing our old garage door before the wife wanted to change it to a newer design. It was the one with the 5 or 6 shorter rectangle insets (or whatever they are called). It did have some texture to it, but not much. I sanded, TSP wipe, and then used a quality outdoor primer and paint. To keep it from drying I painted the top section first and got into the crack a bit and painted the second section. When I was close to the joint, I would roll the door up a bit so the joint was open. I did it in 2 days. Top 2 panels one day and bottom 2 the next day. Did it in the summer so it would dry pretty much that day. It did stick a tad, but not enough to keep the door from opening. If I were to do it again, I would let it dry for a few days before moving it again. Didn't want to leave the door open all night when I painted it.