How long does it need to last?
If it's only temporary, get some GOOD exterior latex primer, I suggest
Bulls Eye 1-2-3 by Zinsser.
Give the foam an initial coat with just the primer to seal it down, and provide 'bite' for the next layer... Next pour some of the primer out into a bucket and mix in some
pre-mixed spackling compound the amount you mix in will be determined by how much detail you need to preserve in the final piece, I would start with a small batch of 50/50 and go from there, if you use a small roller to apply you can get a nice uniform slightly bumpy surface... The spackle will make the paint much harder so you will get a hard shell on the foam... Now you can primer it again or go right to the final paint and coating from that point, make sure you seal down the primer/spackle coat with another coat of paint so water doesn't leach into any exposed plaster on the surface...
This is only for a temp display, not one that is expected to last for long in rain and humidity...