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You should really trowel whatever you are using evenly with a fine notched trowel to help even bonding. I guess it depends a little bit on how flat the plasterboard is, is it fixed to studs or loose sheet. What you are doing is a kind of chunky veneer and the glue I've had success with in the past is contact adhesive. You might need to prime the ply to avoid it sucking up all the moisture. Usually you need a roller of some sort to flatten it out.
What grab adhesive do I want to bond 6mm ply to plasterboard? Screw fix has about 40,000 products all claiming to stick this to that, but does anyone have any real world experience?
It’s 6mm ply so I don’t want anything too thick that won’t compress/flatten easily.