Shot in the dark?

Last year I bought some plaques off E-bay made from plaster moulds for justben of Athena, Aphrodite, and Apollo. We have wanted to hang them up in the house but an unfortunate drawback from them is that whenever you or anything touches them, you get a white powdery "plaster dust" on you/it.

I mean, we have considered just hanging them up anyway and just brushing off the walls or whatever, but we wanted to first pose the question if anyone was familar with any sort of spray or finish we could put on them to keep them from doing this?

I tried to do some internet research on the subject, but it's been all dead-ends. Any ideas?