You’ll want to wash (soap and warm water) and wet-sand the resin before applying any paint or primer. This is good general practice for all resin items- it will remove any mold release and help prep the surface for paint.
I do have one of those shells (purchased from another board member) and I’m not sure what the original maker painted it with. Appears to just be ultra-flat black paint. Whatever it is, it should be sanded down/removed as best as possible, since as you note the adhesion is very poor. When you wash it, you can use an old toothbrush to get into the cracks.