I've found it peals off but maybe it's the cheap Pva I used. I used wickes plaster sealer over the last lot of board as just happened to have some, then a Pva with no problem. I really liking gardz as a problem solved sealer at the moment
I am guessing from the size of the vent the log burner will be over 6 bt u and from the flue pipe sticking through the wall that an insulated clue is not being fitted so, given how close that flue position is to where the fire will end up it WILL crack around the flue. I am not talking small hairline stuff either. Baffle plates fitted around the flue sometimes hide them .
Pboard won't take the heat. Neither will skim. Or anything gypsum based.
Need victas stuff.
Or just get it tiled.
On a side note, you can skim cement board if gritted. Had to do it in a bathroom once where shower area had been cement boarded wider than enclosure. Blended to plasterboard one side and lime float the other no problems.
There must be 15+ primer sealers on the market. We are all never going to agree on which is best for what. Guess what ever works for you. Blue grit. BG do various sorts. Sbr. Pva. Zinnser make a few gardz being one I use. Acrylic primer undercoat for wood and plaster. Wickes do one similar to artex sealer. Artex Sealer if you can get it. Dulux plaster sealer blue. That green rubber stuff called Cementone Plasterers Stabilising. And the list goes on.