Zep has not been marketed for reviving old neglected fiberglass as long as a product called PoliGlow, which is the same******in a different bottle. As Weldon has stated it will eventually fail and start to peel and/or turn yellow. The only way to remove it is either heavy sanding or scrubbing or with a nasty chemical remover that Poliglow sells for just that reason.
Try Googleing Poliglow instead of Zep and see the horror stories. If your ride is so far gone that red fiberglass looks pink/orange the stuff will make it look pretty good but for something I want to keep there is no way I would use it.