I've never tried WD 40, so I don't know if it will work, too. On the other hand, I don't know why it wouldn't.
I add some yellow (waterproof) carpenter's glue to my plaster of Paris, after it's mixed, to make the casting stronger. It seems to make the details hold their edges a little better. I just have to make sure my mold release on the face of the first, sealed half is good, so the second half doesn't glue itself to the first half.
After each half is cured, I brush on two coats of D2T epoxy, thinned with denatured alcohol.
Any iffy spots get reinforced with some super glue.