Q: I’m building a retaining wall using Woodland Scenics’ cut stone castings and want to attach them to extruded-foam insulation board. What is the best glue to use for this project? — Alan Cox
A: The Woodland Scenics cut stone retaining walls are Hydrocal castings, so you’re going to want an adhesive that will work with dissimilar materials.
However, since you’re going to attach them to extruded-foam insulation board, you’ll need an adhesive that’s foam safe. I’d recommend Loctite PL 300, a latex water-based adhesive designed for attaching foam board to other materials. The adhesive sets in 20 minutes and requires seven days to fully cure.
If you can’t find PL 300, there are some products from DAP Global Inc. that will also work. First is DAP Foamboard Construction Adhesive. It’s designed for use with extruded-foam insulation board and is an instant-grab adhesive. This means you won’t need as many clamps or fasteners to hold the retaining wall castings in place while the adhesive dries.
Next is DAP Alex Plus, an acrylic latex caulk that is infused with silicone. It’s available in clear and white, though the color doesn’t matter all that much because the caulk will be hidden between the castings and the foam.
The third product from DAP Global Inc. is Dynaflex 230. The elastomeric latex product can be used for caulking and sealing. It’s available in eight colors plus clear. When cured, the waterproof sealant is mold and mildew resistant. It also notes on the manufacturer’s website that the “high flexibility sealant accommodates joint movement caused by temperature and humidity fluctuations without cracking.” All of the DAP products listed here can be cleaned up with water.
One other product to consider is Liquid Nails Projects & Foamboard interior construction adhesive from PPG. As the name suggests, the adhesive is safe to use on extruded-foam insulation board. Tips for how to glue dissimilar materials are listed on the product’s webpage.
You’ll want to avoid using solvent-based caulks and sealants. Though they’re safe to use on Hydrocal, they may etch or burn through the foam.
If you plan on weathering the cut stone retaining walls with Woodland Scenics Earth Colors liquid pigments or other stains, do that first. If the adhesives noted above ooze onto the face of the castings, they may seal the Hydrocal, preventing the porous material from taking pigments and stains.
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