I will not be much help on the garage doors, however I just redid about all of our exterior doors. What I did was scrub them down and then lightly sanded them in tough spots. (Are you planning to take off the current latex paint before re-apply a new color? If so, there is a whole process for that) We used Behr Premium Plus Ultra Exterior Flat and have loved the results and especially the price. I am no expert though, just hope I could be of some help! Best of luck and have fun!

Here is a link that might be useful: Behr Premium Plus Exterior

I wasn't planning on stripping off the old paint. Its not chipping or anything, I just need to change the color. Right now the door is white, shows all the dirt and I want to paint it gray. So, I think I can go over the white paint, after a good scrubbing. Since the garage door paint is worn in some spots, I think primer is indicated, but I'd like some opinions as to which primer would be best.

but I'd like some opinions as to which primer would be best.

maybe not the BEST but pretty good

Zinsser 123

Thanks, Christophern, pretty good works for me. I'll check it out today......