Why only a few minutes after I mix the plaster it goes grainy and is impossible to work or smooth? Do I need to mix the plaster with a product or prepare the wall a certain way?

firstly look at the expiry date on the bag of plaster? and once opened it will deterate within a few days if the air gets into the bag both making it grainy? if its in date and unopened try wetting the area with a watered down pva solution we usually do 5 parts water to one part pva let it dry in usually a about a hour on a porous surface then try again

What are you doing here? Paris is my favourite place in the world!! Try sealing the areas to be plastered with a 3;1 water;pva mix. this will kill the suction and give you more time to trowell. Mark

Hi Its sounds like the plasterer is out of date that your mixing up. Prestonnett Beisser nobody in the uk even stocks it unless you order in, you can get fiberglass scrim 1mx50m, but if you turn up on site with that and start covering the walls in it you will be out of pocket on a skimming job before you ever start. i would say go with advice from D.R.S maintenance that’s your best bet, on a damp day our uk gypsum based plasters take longer to go off. all the best to you

try using a relatively clean bucket and also make sure its clean water your using, make sure paddle mixer is clean and free of any flaking plaster remnants, check the plaster is in date and also its not a damp and an open bag thats a few weeks old that airs got to.. Pva the ceiling 3:1 mix to seal the ceiling let it dry 15-20 mins then Pva again let this go to a tacky state your good to go. if there is signs of distemper on your walls or ceiling plaster scrape these down to remove the loose and flaking paint wash walls with sugar soap allow to dry prior to Pva ... 1m wide scrim cost you a fortune in the uk...