Im sure a decent floor paint like coovar will do the job providing that it can key to the surface of the glass , i only mentioned it as i have a few old timers who are paint it if it dont move types
Some of the works have blistered , some have not even taken , as long as the surface is bone dry and lightly sanded you will stand a good chance of waterproofing it .
But all said and done the proper top coat for glass is gel coat and that is the way it is , otherwise you would have superyachts sinkin all over the place cos some geezer emulsioned them with Crown