It is the catalyzation process that gives the paint its hardness. That said, since I don't know what paints/coatings you already have on the parts, I'd suggest proceeding with caution. Some paints aren't compatible and may cause underlying layers to peel, lift or wrinkle. Some improvement in paint hardness may come from just letting it continue to dry. Most of what I know about paint chemistry comes from having painted a few cars. In that case the rule of thumb is pick a paint system and stick with it throughout the process. As long as the surface of the plug is hard enough to stand up to the waxing and application of mold release and your mold making process doesn't press hard enough on the plug to deform anything the mold surface itself shouldn't have to be diamond hard. The fact that you have produced what looks to be a sophisticated plug with a lot of compound curves leads me to think that you have researched the process pretty thoroughly. I'm looking forward to seeing your next steps.