Subject: cobblestone making questions

I've bought a 30 pack of your 6x6x1.5 inch cobblestone molds, and just recently bought 8 sets of 6 of your 6x4x1.5 molds. I've made probably 600 rocks with the first set of molds. It took me a while (maybe 100 rocks) before I got air-hole free rocks. But recently I've begun to get a few holes near the corners of the rocks. And the corners don't look as smooth as the middle. I've attached a few pictures as examples. Any ideas to get these to go away? I have noticed that the molds are dirtier near the corners (maybe I don't put enough mold release on the corners).

I haven't used any sealers, is there any sealers that I would use after all the rocks are made and installed? Or should I have used a sealer while making them?


Hello Kevin,
By the looks of the photos you provided, (thank you), I would say that you probably need to clean out the corners of molds of dried cementitious material. Corners need to be soaked and cleaned periodically of built-up concrete. It sounds like the cement in the corners may be absorbing your mold release oil, and causing the water in your concrete mix to be displaced when you make your pours. The oil will actually "push" the water in the mix out, (away from the corner), and cause the void which then becomes an air pocket, thus creating a bug hole, or air hole in the corners of your stone. The residual cement in the corners also leave your new stone with the rough texture I see in the photos, as it covers the smoother mold surface that the fresh concrete should be duplicating.

This can even happen in a new mold if you leave too much oil in the corners. Another thing that can happen is that if the molds lay around for a long period before pouring your concrete in, the oil can accumulate in the corners of the mold. In certain "flat" molds, the oil will sometimes migrate to the mold corners, as most molds are ever so slightly raised in the middle of the face of the mold. This does not normally affect the stone produced, as the concrete is heavy enough to flatten the center down. But the slight "crest" in some molds will allow oil to move outward from the center prior to pouring in the concrete.

So, clean the mold corners with some soapy water, or even scrape out the excess cement residual with something like steel wool or a small teaspoon or butter knife blade after letting the molds soak. Then be sure to apply mold release to the corners, but don't leave too much in there. That should solve your problem.

As for sealing... you can wait to apply a sealer after installation if you will not be grouting or using mortar between the pavers or stone during installation. If you will be grouting, I would suggest sealing before grouting to make clean-up of the stone paver possible. If you want a natural, no sheen look, the best sealer to use would be a penetrating sealer like our StoneKote. If you buy a sealer locally for your stone, be sure that it is designed for stone. We've had a few customers try using a penetrating wood sealer like Thompson's, but it is not designed for concrete. There's no sense wasting your money on it. It may be great for wood, but by the sound of our customers' input, I question its use on concrete or stone.

Customer Problem - 
Air holes, bug holes in Concrete Stone and Tile They Made 



Thank you so much for the prompt reply.

I tried using vegetable spray from Sams club and put a light coat then wiped it out.  This worked perfectly!  Previously I used an old paintbrush to apply the vegetable oil and would wipe out any excess but I suspect there was still too much residual oil in the mold.

Your tip for repairing the bug-holes was also spot on; I was able to repair about 75 stones with the excess from pouring my last batch. This will be a huge timesaver since I can now use ones that I was going to use as fill.

Thanks again!

On Wed, Jun 9, 2010 at 11:18 PM, Olde World Stone Molds <>
Another one of our customers had a similar problem with bug-holes, and we responded with basically the same information to the above... His June 9th response:

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