I wouldn't use ZEP or other high gloss product on the roof - just too slippery. I DO get on the roof occasionally and want to minimize the risk.
My 9 year old fiberglass roof is painted and for the past year or so it has begun to shed oxidized paint (chalk) in the form of white streaks down the sides after rain or dew. I plan to re-paint it, but in the interim have been applying an inexpensive floor wax (Mop n Glo or equivalent) to reduce the chalk streak problem. A coat of Mop n Glo lasts a couple months and leaves no residue. No worries about waxy build-up, cause it's just plain gone!
Prep work is simply to wash the roof, but I wash with a strong solution of detergent and bleach and use a medium bristle brush to get crud out of the pores and crevices. The chalking comes back in a few weeks without any floor wax, but the wax extends the time by 5-6 weeks.
I plan to wash and paint the roof this fall, weather permitting.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL