Sealer to external wall paint

Hi. I have a flat roof connected to the main house. The walls are icy with thin render - the final coat being a 2mm acrylic render. There is drop of water coming into the room below when raining for a while. I have come to the conclusion that the water is entering through the render and down the back of the lead which eventually means drops into the wardrobe . I want to paint on a sealer to the external surface to test this theory . The sealer would need to be transparent and I would need be able to paint over it as we want to repaint the house next year.

can I ask you to suggest some products please. I live in the west of Ireland(Not coastal) , rain is something that is common on a daily basis.

Thanks