Your problem will not be crystallization, it will be getting the epoxy to be completely clear so your dark color can shine through. Epoxy is a great adhesive, and your bond will be less strong than it would be to unpainted foam, but still better than poly resin.
Make sure the paint is completely dry, wait at least a day (maybe two), since you painted it thick. If this is your first lamination with epoxy, do not get your hopes up; dark colors can be a bitch to laminate over with epoxy. You have to use more resin, and work it as little as possible. No ‘stretching’ the resin from one place to the other (like using the excess from one area to saturate another). No catching drips from one place and putting it somewhere else, or you will get a cloudy, frothy air filled mess mess. I don’t paint on the foam anymore because of this.
To tell you the truth, though, no matter how the laminate comes out, you can always paint on top of the sanded hotcoat the same pattern you used on the foam. With only a gloss coat on top of it, the colors will be brighter and more vibrant on the hotcoat, you just have to be very careful when wetsanding and polishing the rails…you could also sand some of the paint off the blank, and use the same pattern on top of the hotcoat if you are worried about adhesion to the blank…
6 oz. cloth will come out clearer tha 4 oz if you have that option. Use a bit more resin and wet the laps with clear resin out of the bucket instead of resin scraped from the flats. This will also help the rails to come out better with that dark color.