Hi... is it Ok to lay a 2mm coat of Thistle Undercoat plaster onto a wall that has already had a 4/5mm coat of Thistle Hardwall layed?... or what is best option if extra 2mm needed prior to applying 2mm Multi Finish coats? Do I need to obtain more hardwall or apply more layers of Multi finish? Hardwall applied today and has been given key. The reason I ask is Hardwall run out and I have a bag of undercoat.