Straightforward answer is yes, provided the epoxy is prepared by roughening well. It is an odd and uncommon specification though. Either specify a full alkyd system or a full epoxy system. One last thing, zinc phosphate does not contain free zinc, so saponification is not a concern.
The reason for over-coating the epoxy with zinc phosphate was that one of the project asked for 2 coats of alkyd-based primer with 40 microns per coat and 2 coats of synthetic enamel topcoat with 40 microns per coat with total dft of 160 microns. To achieve this with an alkyd-based enamel is somewhat difficult, so I felt that getting 120 microns with an epoxy-based primer in one coat and one coat of enamel with 40 microns will give me desired dft, and since I am using an epoxy primer, it will provide far more corrosion protection than an alkyd-based primer.
Alkyds with metallic zinc saponify, which is why you cannot use alkyds on zinc silicates, but this reaction cannot occur with a zinc phosphate so no problems overcoating. As mentioned, there are many formulations of alkyd primer containing zinc phosphate, though, as previously asked, why would you want to use an alkyd over an epoxy when there are so many superior options?
Yes, providing the epoxy primer is prepared correctly- professionals will know how. For exterior durability, I would have to ask why an alkyd would be chosen, when 2K polyurethanes offer superior gloss, durability and color retention.
Yes, you can apply an enamel-based alkyd topcoat over an epoxy, but you cannot apply epoxy materials over alkyd.
You can apply an alkyd over the epoxy primer if it is exposed to sunlight for UV not for corrosion protection, but the life cycle is short compared to an epoxy or polyurethane topcoat.
Good question. I have no previous experience of this application case, but I know that zinc and alkyd are not compatible. In the market there are alkyd primers modified with zinc phosphate pigment, which makes me think that an epoxy pigmented with zinc phosphate can be finished with a good quality alkyd enamel.