How do you seal a styrofoam made waterfall?

Answer 1

I can tell you from experience, that if you use spray sealer over a grout-covered, styrofoam (white compressed bead-type is what I used) waterfall that it will discolor shortly after the water is turned on.

I constructed my waterfall from white styrene bead-type foam sheets. I gave it a coat high-hiding water based priming paint (it contains micro-silica bits that increase the opacity, I suppose. It was safe for children, so it's safe for my critters.) and let it dry for a couple of days as I did other things.

I then gave the feature 3 coats of grout (tan colored - colored by the mfr.), letting each coat dry 24+ hrs for each. I applied acrylic paint -very, very dilute and with a spray bottle, wiping off the raised areas to simulate sun bleached rocks.

Then I found a can of craft sealant. This should help to water proof it, thought I. The dried water feature then had a semi gloss finish. I coated the water path with silicone seal and let it cure.

2 weeks later, the whole feature was covered in hundreds of white spots. Water percolated between the styrene beads, to be trapped between the sealant and the grout. To fix this, I had to repaint the whole feature using acrylic craft paints. 1 month later, there are no spots. [In retrospect, in the future I will only use closed-cell foam for water features. Then the water can't permeate to the surface of the foam from under the grout.]

TIL: Do not apply a spray sealant to foam & grout water features.