Doors – foam roller vs brush for fiberglass door

This article provides a pretty good run-down.

1. Yes you should clean: soap and water or any other mild cleaner is good.

2. You may not need additional primer, but you should fix any dents/scratches -- they will stick out after you put the new paint on. Auto-body filler (eg Bondo) works for dents and deep scratches. Before you prime, the whole door should be lightly sanded using 120-150 grit sandpaper to roughen the surface, and then wipe the whole door down to get rid of any particles from sanding.

3. Use an oil-base primer, and again, be sure to lightly sand first.

For getting paint off a door, I highly recommend using Citrus Strip. We tried it on our old wooden door and it worked great, taking off multiple layers of paint. It doesn't work as well under a lot of sun and heat, so I would recommend either taking the door off the hinges or erecting some sort of tarp to block the sun from hitting it directly. Then get a paint scraper,and it will come off pretty well.

For painting the door, they make small rollers (cabinet rollers) that we also used on our door, and it worked out very well. They are only about 4" wide. I believe we used a latex paint, but I will check the can when I get home.