A Major (And Much Improved) Design Change To My Silicone Faux Stone Molds (Plus, A Progress Status Update)

Today is the day, y’all. It’s the day that I’ll finally be able to start attaching my faux Austin stones to the skirting area of the front porch. We’ll be having a beautiful day here in central Texas (hot, but at least no rain in the forecast), and I have all of the big stones ready and waiting, and about half of the small stones ready to go.

When I originally made the mold forms for the textured silicone mats, I needed something to place under the silicone mats to hold the edges straight and give the mats support underneath. Otherwise, I’d end up with wavy edges on my stones…

I ended up using the type of sand that you would use underneath paver stones. I already had the sand on hand, and it seemed easy enough to lift the edges of the silicone mat, drop some sand underneath, and then adjust the silicone mat so that the edges were straight.

Once I had the mat at the right height all the way around, and completely supported underneath, I vacuumed away the excess sand and then caulked around the edges of the silicone mat where it met the PVC boards that I used to make the molds.

I thought that would be mostly a one-and-done process. I knew that in order to demold each stone, I would have to remove one side of the mold, so I thought that meant I would just have to re-caulk one side of each mold before pouring the next stone. That turned out not to be the case. As I removed each stone, it pulled the silicone mat up a bit along with the stone. And that dislodged other areas of the mat. Once those other areas of the silicone mat were pulled away from the caulk, sand would come out. And because I was using silicone caulk between the silicone mat and the PVC boards, which constantly remained sticky, the sand would get stuck in the caulk and create a huge mess.

So I was basically having to go through half of the mold-building process before each new pour — adding more sand, shifting the sand around, getting the silicone mat just right, clean up the excess sand, and then re-caulk the whole thing. That was so time-consuming!

I finally decided that the perfect product for that is Great Stuff expanding spray foam. It would fill in those areas, and once it cured, it would hold its shape. I specifically chose this brand because, after a quick google search to find the fastest curing spray foam available at big box stores, this was the one. It sets within about 15 minutes and cures in an hour.

So I took one side of each mold off, emptied the sand, and got each mold cleaned up by removing all of the remaining sticky caulk with sand stuck in it. And then I lined up the silicone mat along the straight guideline that I had drawn on each one indicating where the edges of the silicone mat needed to be. I held it in place with one hand as best as I could, and then I sprayed the spray foam between it and the bottom of the mold box. It did its thing and expanded and came out of the open side. While it expanded, I just kept pushing the silicone mat into place to make sure it stayed lined up on the mold box.

After it stopped expanding, I scraped off the excess that had come out the side…

And then I screwed that side of the box back on and made sure the mat was still lined up on it’s guideline. I had to push it down more in some areas, and that caused the spray foam to come out between the box and the mat. But I just left it and let the foam dry completely. Once it was fully dry, I cut away the excess foam, re-caulked, and was ready to pour the next stone.

This made the whole process so much easier and cleaner. I no longer had to deal with shifting sand. I no longer had to deal with basically remaking the mold again between pours. The foam held its shape, and because it’s very sticky when it’s wet and had grabbed onto the silicone mold, it also held the silicone mat in place as I demolded the stones.

I was able to lift the stones off of the mat while the mat stayed in place on the other three sides.

The stone came out very cleanly and three sides of the mold remained completely intact.

Then I just had to put the side of the mold back on, re-caulk one side and two corners, and I was ready for the next pour.

So while I’m not quite finished pouring stones, I am ready to get started attaching them to the skirting area of the front porch. I hope to have some progress to show you tomorrow!