Porous silica ceramic substrates were coated with silica sols via dip coating to obtain waterproof performance. Silica sols were prepared through acid-catalyzed hydrolysis and condensation of tetraethyl orthosilicate. The gelation temperature, ratio of ethanol/TEOS, times of coating, and sintering temperature have been studied. The sample with the best waterproof effect was prepared by 5 times of repeated coating using a sol prepared at 25 oC with an ethanol/TEOS ratio of 7 and sintering at 550 °C.