I have an Edwardian Terrace house where the front door is set back about 2ft from the front facia and originally this was an open storm porch area. It looks like in the 1950s a secondary vestibule" glass door was put in to enclose the open storm porch. In this enclosed storm porch area, when the house was renovated a few years back, the Builder replaced the old lathe and plaster walls (breached fire Regulations for HMO) and put in 30 minute fireproof Plasterboard on one side of this enclosed small porch area. There have been no problems since. I now want to remove the 1950s vestibule door and bring the house back to its original open storm porch. I therefore need to make the plasterboard (now painted) as weatherproof as possible. Can anyone advise as to the best material to use for this? Would the use of exterior paint be sufficient? I saw that Tanking is the best possible way to waterproof plasterboard but its not clear whether tanking solution could be used over the existing painted surface.