The requirements for interior paint are significantly less than for exterior. The big problem in developing exterior paint is resistance to UV light
. Interior paint is not affected by much UV.
So using a paint like Interlux Brightside is great, but rather expensive considering where it is being used.
Zinzzer 1-2-3 is a shellac dissolved in alcohol. It sticks to just about anything, and just about anything sticks to it. It also builds up quickly, dries quickly, doesn't smell too bad and is impervious to moisture. So it is a great primer that doesn't cost much and can be applied with a brush or roller.
For the top coat any good enamel would be just fine. I used a floor type enamel with great results. It dries a bit slow, but since it was a warm summer day it worked out fine. By the time I finished a coat and got a cup of coffee, it was not quite tack free and the next coat just melded right in. After three coats the dingy greyed ensolite was a nice bright white. Three years later, it still is and is much less likely to get mildew in all the little wrinkles.