Today is the day. It’s the day I can finally start attaching my faux Austin stones to the porch baseboard. We’re going to have a beautiful day here in Central Texas (hot, but at least no rain in the forecast) and I have all the large rocks ready and waiting, and about half of the small rocks ready to go.
I only have half the small stones so far because I need so many of these two sizes, but I only have one shape of each size. While I only need three or four of the large stones each to complete the porch base area, I need many more of the two small stones – nine of one and ten of the other. And the casting, curing and demoulding of these small stones takes just as long as the large stones. But at least I have enough to start on the baseboard while I continue to pour the remaining small bricks I need to finish the entire baseboard area.
I wanted to show you a big problem I ran into when designing my silicone molds and the changes I made to them to make casting these bricks much quicker and easier.
When I originally made the molds for the textured silicone mats, I needed something to put under the silicone mats to keep the edges straight and provide support to the mats underneath. Otherwise I would have wavy edges on my stones…
I ended up using the type of sand you would use under pavers. 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 the edges were straight.
Once I had the mat at the right height all the way around and had it fully supported underneath, I vacuumed off the excess sand and then caulked the edges of the silicone mat where it met the PVC sheets I used to make the molds.
I thought this would be mostly a one-time process. I knew that to unmold each brick I would have to remove one side of the mold, so I figured that would mean I would only have to re-caulk one side of each mold before casting the next brick. Turns out that wasn’t the case. As I took out each stone, he pulled the silicone mat up a bit along with the stone. This caused other areas of the mat to shift. Once these other areas of the silicone mat were removed from the caulk, sand came out. And because I used silicone caulk between the silicone mat and the PVC panels, which remained sticky all the time, the sand got stuck in the caulk and made a huge mess.
So before each new cast, I basically had to go through half of the mold making process – add more sand, move the sand, get the silicone mat just right, remove the excess sand, and then reseal the whole thing. That was so time consuming!
I knew there had to be an easier way and a better solution than using sand under the silicone mats. I just needed something to fill the gaps between the silicone mat and the bottom of the molding box, but it had to be something that would dry and hold its shape instead of shifting like the sand did every time I unmolded a brick.
Finally, I decided that the perfect product for this was Great Stuff’s Expanding Spray Foam. It would fill these areas and hold its shape after curing. I specifically chose this brand because after a quick Google search for the fastest curing spray foam available at big box stores, this was it. It sets in about 15 minutes and hardens in an hour.
So I took off one side of each mold, emptied the sand, and cleaned each mold, removing the remaining sticky caulk with sand stuck in it. And then I aligned the silicone mat along the straight line I had drawn on each one that indicated where the edges of the silicone mat needed to be. I held it as best I could with one hand and then sprayed the spray foam between it and the bottom of the molding box. It served its purpose, expanded and came out of the open side. As it expanded, I simply continued to press the silicone mat into place to ensure it stayed aligned on the molding 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, making sure the mat was still aligned with the guide line. I had to push it down harder in some places which caused the spray foam to leak out between the box and the mat. But I just left it and let the foam dry completely. Once it was completely dry, I trimmed off the excess foam, re-caulked it, and was ready to pour the next brick.
This made the entire process much easier and cleaner. I no longer had to deal with blown sand. I no longer had to worry about basically renewing the mold between castings. The foam held its shape and because it was very sticky when wet and clung to the silicone mold, it also held the silicone mat in place when I removed the bricks from the mold.
I was able to lift the stones off the mat while the mat remained in place on the other three sides.
The stone came out very clean and three sides of the mold remained completely intact.
Then all I had to do was put the side of the mold back on, reseal one side and two corners and I was ready for the next pour.
I wish I had thought of doing it this way from the start, but of course DIY is all about trial and error. And now if I ever do another project like this, I’ll know how to properly (and much more easily) build a mold out of a textured silicone mat. And who knows? I may use these shapes for another project in the future. I’ll definitely be keeping them just in case I want to do something different to match the Austin stone in our house.
Although I’m not quite finished pouring the stones yet, I’m now ready to attach them to the porch baseboard. I hope to show you some progress tomorrow!
