Good Morning! I took out the water tank to make it clean. The tank is fiberglass. The plumber who is going to do the job has asked me 4 liters of polyurethane membrane paint to paint it inside and out, at least that says in the budget and in the list of materials that happened to me. Is it correct to paint it with a polyurethane membrane? The tank was never painted. Of course, thank you very much for the advice you can give me. Regards!!!