How to Pour Concrete, Cement or Plaster Into Uneven or Round Bottom ABS Plastic Molds

The easiest way to pour into uneven and round-bottom concrete molds is to place them into a bed of level, moist sand. This technique will support the sides and hold the mold level until it sets. You should still cover the poured molds with plastic to aid the hydration process. And don't forget to use mold release in the mold. If you want to get varied coloration, you can sprinkler in some powdered colorant, shake it around a bit, and dump out any excess. Experiment with some different color combinations and techniques until you get the results you are looking for. Sprinkle colorant on top of your concrete mix prior to scooping up the mix to fill the molds, as well.
Click on CONCRETE COLORING TECHNIQUES for more detailed instructions.

I suggest you start by building a “sandbox” out of 2”x4”s. Fill it with fine sand and level it off as much as possible. What I do is use the level edge of another 2x4 to screed the top of your box. Basically, you over-fill the sandbox with sand, and then run the 2x4 along the top edge of the box to smooth and level the sand within. You will then have a nice level sand base to start with once you do this procedure. Or for just a couple of molds, you can use a small “pile” of sand, and put a level across the top of the mold after setting it in the sand to be sure it is level. Using a sandbox makes the leveling almost automatic.

Before you begin your pouring session, dampen the sand slightly to give it a bit more body. You may want to push the molds gently into the sand to create a cavity before filling the mold with concrete. This will facilitate a faster and easier insertion of the mold once it is filled with concrete.

Push the mold into the moist sand. Pour the concrete mix into the mold. Lift it out of the sand so you can vibrate it a bit to release any bubbles, and to compact the solids to the face of the stone. Reinsert the mold into the sand cavity, and level. Cover all of the poured molds with plastic sheeting once filled, to aid hydration and the curing process. Leave them undisturbed for at least twelve hours, depending on the temperature. If you are not in a hurry, leave them undisturbed for a minimum of 24 hours or more. The longer you leave them in the mold, and covered with plastic… the stronger they will be, and the easier they will be to demold. Leaving the stone in the mold for a few days does not present a problem either, and is actually beneficial. Click here on MAKING CUSTOM STONE for more in-depth information about making concrete stone and tile.

If you have a number of molds, and are doing a few pours, you may gently lift the molds out of the sand once they are hard, (a few hours), and you can then pour additional molds using the same sand cavities. Be sure the concrete has hardened sufficiently though. Stack the filled molds (gently) and cover the stack or pile with plastic to aid the curing.

Another option is to build a frame out of 1”x3”or 1”x4” lumber to hold each of the various size molds. Basically, you build a “box” that the mold will fit into, but which allows the mold edge to lay on top of the frame to help support the mold. I like the sand system better, and it is actually much easier once you get used to it. One big advantage to the frame system is that you can stack the molds on top of each other as they are being filled. A piece of plastic between each layer seals the heat and moisture in, and aids in the hydration and curing. Obviously, it is your choice though.

When the stone is ready to extract from the mold, lay it face up on a flat surface. Apply slight pressure to the mold edges, and the top of the mold. Do not push in too hard, or you may crack the mold edge. The stone will usually drop right out of the mold as you lift the mold off. After extraction, be sure to cover the stone with plastic. This will aid in the curing process. You DO NOT want to “dry” your stone quickly. Keep it wet for as long as possible, as this procedure will give you a much stronger stone.

After installation of your stone, you may want to apply a penetrating sealer such as our Olde World StoneKote Sealer. This will help protect it, waterproof it, and help in freeze-thaw environments. You may also use a matte or gloss sealer if you desire, in lieu of the recommended “flat”, or “non-sheen”, natural stone finish. Click here for more information about CONCRETE SEALING TECHNIQUES”.

Should you need molds or moulds, colorant, concrete sealer, stains, or other concrete, cement, or plaster supplies for your handyman or home improvement project, please visit our catalogue website at or our shopping cart website at  to get ideas.  If you are interested in starting a concrete stone and veneer, or concrete and cement paver, brick or tile business, visit our Olde World Stone & Tile Business Opportunity website at for full details, instructions and various stone making and other concrete product making packages.  We also offer D-I-Y Packages in our catalogue and on our shopping cart websites.


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