Hi
@theclemo - this is a tricky one – there is no single cause so no “magic bullet” to stop it. It tends to be more likely if:
1) The site weather is cold
2) The plaster is mixed much more than necessary
3) More water is used during trowelling up
4) The final trowel is done with a high trowel angle (some call this a “cutting” action – the opposite of a flat angle)
The best way to avoid it, e.g. in cold weather, is to try to work neatly and get the surface flat early, so there is less need to move material around later – it’s this that seems to make it show up. Hope this helps!