No plasters are guaranteed outside, as it is not weather resistant. This is because it is porous, thus making it vulnerable to damage from water ingression from the elements such as rain and frost.
However the harder the plaster the longer it will last. Also by mixing a plaster like Crystacal R Plaster with Plaster Polymer instead of water, you are effectively sealing the plaster, so protecting it from the elements that would normally attack and damage the plaster. Another option is to entirely seal the plaster with an exterior sealer or varnish, thus stopping the water ingress.
The closest product to plaster for outside use is probably Jesmonite AC730 or Fibrocem, or other cement based materials.