Try washing it a few times on a hot setting with detergent and a bit of fabric softener. This should cut down on the fabsil coating letting the cotton absorb the water. You could also leave the smock soaking in a bucket of water and a cupful of detergent since i know this takes away the coating on the new ones, don't know about ventile though.
Tonyuk
I wouldn't put any fabric conditioner in, it changes the way the fibres absorb water and stop them from swelling in the damp. That's the same reason that you don't use fabric conditioner on towels, it makes them less absorbent.