Sealing styrofoam waterfall

[kressg23]

Hi All,

I've searched and searched and can't find an answer to this specific question, even through Google. Hopefully someone here can please help. Is it possible to use waterproof (siliconized) grout to cover over a styrofoam waterfall? Would it still be necessary to soak it in vinegar if after the grout cured and was painted, a very thin layer of clear silicone was applied over the entire piece as a sealer? This water feature will be added to a 40g tall vivarium that will eventually house dart frogs. Thanks in advance and I truly apologize if this has been answered elsewhere and my search methods weren't good enough to locate it.