In one of our installation in lonavala (very high rain)we hv a sloping cement fibreboard roof . We are doing gysum false ceiling beneath. There is enough gap between the cement board roof & gypsum false ceiling . We want to make the gypsum board water proof . There is enough space to paint the gypsum board from inside that is the inner face between the main roof & gypsum board ceiling where we feel some rain water may seep . The gypsum false ceiling is also sloping for any small quantity of rain water to flow out. Which water proof paint can we apply . Can we apply weather coat of berger or raincoat of dr.fixit . Does this paint interfere with the gysum material .