As mentioned above Partall #2 wax is the best. For a plug I will apply 10 coats waiting the full half hour between coats. I will also apply 10 coats to new molds for their first pull. You mentioned that you used primer for your mold. Perhaps others have had success with it... but it is much softer than using a tooling coat. The stuff was actually invented for this purpose. If you dont want to spend the money on proper epoxy tooling coat then you could do as mentioned in the above post.
I've tried paints before as a tooling coat but never had any success so I now stick with proper epoxy tooling coat.