Polycell Expanding Foam 750 ml. This water resistant expanding foam from Polycell has been specially designed for filling sealing & insulating most building materials around the interior & exterior of your home. Simply apply & leave for 20 minutes to dry. It comes in an easily storable & transportable aerosol & expands to the exact size of the cavity you need filling. If you're using it during redecoration projects it's sandable & paintable making it perfect for giving a smooth professional finish once dried.