I'm making some styrofoam ledges for my wtf viv and I have everything figured out except what to use to seal it. I carved styrofoam (I used the blue insulation boards because I had some on hand) and covered it with a few layers of sanded grout. Now I just need to make them waterproof, but I've found a lot of different suggestions and conflicting information on that part. Has anyone made these before and can suggest a product?