You have to understand that the softer the rubber, the more flex in it, and thus the more likely you will get microfractures in any coating. I have had very good luck coating EPDM and buna-n type rubbers with the following process. Don't deviate or skip any steps.
- clean thoroughly with acetone or MEK.
- mechanically abrade with 180-240 grit (higher for smoother finish, but less lifespan, coarser for rougher finish but better adhesion) sandpaper
- wipe again with acetone or MEK
- prime with any UMA primer like XIM400
- paint with devthane 379 aliphatic urethane enamel.
Comment about MEK here: It is not good to use MEK in large amounts on plastics and rubber as it tends to break them down, but this is the point. the partial surface dissolution of the base substrate will allow a better chemical bond with the primer. If you want to know more, look up the properties and hazards of MEK.