Hi I have bought from eBay a sheet of Promat supalux board really cheap as I need to line an old fireplace which has a log burner installed in it. I am putting in fireboads as I had the room replastered, but the plaster plastered inside the fireplace which is now fallen off. My query is how to install the board on the existing sides of the fireplace? Not all the old plaster came off. The back already had a fireboad installed.