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How to waterproof Styrofoam to imitate waterfall rocks?


Using Styrofoam to build fake rocks and seal them to allow water to run in falls or trickles. All rubber paint destroys foam (styro). Is there any product that can be used as a primer, or a spray on material that won't destroy foam?

Bob Young
hobbyist - Veradale, Washington, USA


One way may be to "seal" the styrofoam with Artex paint []. This is a sort of plaster paint sometimes used to texturise walls and ceilings. Then waterproof the artex with a polyurethane or acrylic paint and/or sealant. This can also give you the correct colours for rocks.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


I have been wanting to build a waterfall out of fake rocks and cannot find a site selling them and found your question about the paint. I assume you made your own. Do you have any tips for me or a site I can buy the rock?

Melissa Abriani
- Clinton, Indiana, USA

October 19, 2010

I have been building a waterfall for some time now, and the reason it's taking so long is that I am in Taiwan. No one knows what I am talking about concerning materials, because D.I.Y. doesn't really exist here. It's very frustrating to find the correct materials, but they are coming around slowly but surely.
To start off, I bought a big brick of styrofoam and used a knife, safety to cut and tear away with my hands; the areas to make rock type formations. After I got rid of smooth surfaces and created a rock type wall, I bought some expanding foam to create more depth of field.
I am now at the part where I seal the entire structure with plaster paint, but that is where I am stuck. I am trying to find the correct material, but it seems impossible, but it'll come around sometime soon, I hope.
When I am finished, I will show pics on facebook. (Feebie's Pizza Taiwan)

Carey McGregor
- Taiwan

October 19, 2010

Hi, folks. Letter 3295 is on basically the same subject, and explains just about all the alternatives. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

May 22, 2011

I just recently made some fake rocks (to cover some underground water meter openings in my front yard). I made them using an old bucket and spray insulation expanding foam. I just sprayed the foam all over the bucket in an abstract fashion.When dried, I carved with an old steak knife, and when I was done I used a can of spray Kilz primer. Then when that dried I used ordinary spray paint. The Kilz stainblocker/primer can also be used on the white beaded styrofoam also.

Shirlee Schuster
- Greer South Carolina USA
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