Blistering in plasterwork is caused by a high moisture content in the backround and/or over trowelling before the plaster has dried/set.
Blistering in finish plasters
When plastering over backing plasters such as Thistle Bonding or Thistle Hardwall blistering can accrue when the application of finish plasters is to early. You should allow the backing plaster to set but not dry before plastering.
If for some reason you can not apply the finish plaster the same day as floating the wall/ceiling (next day) you should not PVA (prime) the backing plaster, just spray some water or in the instance you are skimming over Thistle Hardwall the next day the wall should be pre treated with Thistle Gyp Prime to control the suction.