Pouring concrete into stone cladding moulds allows you to create panels that look like real stone masonry. You can buy these moulds from a number of suppliers, or you can make your own, using a pourable mould-making compound. Making your own cladding moulds can be cost-effective, because you need to cast several panels even to cover a small wall, and each panel takes three days to fully cure. So the more moulds you have, the faster you can produce the panels you need.
Build a pour box. Cut the board into two, four-foot lengths and two, two-foot lengths. Nail them together to form a two- by 4-foot frame that is six inches deep. Nail the plywood over one side of this frame. Seal the inside of the box with polyurethane spray sealer.
Arrange the stones in the bottom of the box, facing up, the way you would want to see them in the cast duplicates. Place them about as far apart as you intend to install them on the wall. Leave a one-inch margin between the outside edge of the group of stones and the wall of the pour box. Seal the stones with the spray sealer.
Mix equal amounts of both parts of the urethane moulding compound in a clean bucket. Mix enough to cover the stones in the box to a depth of at least one inch. Stir thoroughly until the mixture colour becomes consistent.
Pour the mould compound over the stones in the box. The compound should cover the stones to a depth of at least one inch. If you have not mixed enough, immediately mix more; you have about 30 minutes to add additional compound before it begins to set.
Allow the urethane to cure for 48 hours.
Invert the pour box and shake slightly to allow the urethane mould and the stones to drop out of the box. Remove the stones and wash any residue from the mould.
Wait an additional five to seven days before using the mould to make casts; this will allow the urethane to reach its full strength.
This procedure allows you to make cultured stones that can be applied on a wall individually. If you want to make a casting that will go up as a single panel, you must lay cling film on the bottom of the pour box, and then grout the stones just as you would on a masonry wall; that way, the mould will capture the grout lines as well as the stones. If you wish to duplicate an existing stone wall, use a brush-on urethane mould-making compound. It will probably take several coats of this compound to build up a mould that is thick enough to bear the weight of poured concrete, however.
Make sure you have your stones laid out in the pour box exactly the way you want them, before you start mixing the urethane compound; once mixed and exposed to air, the urethane must be poured within 30 minutes, or it will begin to set. Failure to properly seal the box and the stones will cause the urethane mould to stick to them; this may damage the mould as you remove it from the pour box. Check the urethane compound instructions to see if any additional mould release agents are needed before you pour. Allow the urethane to fully cure for five to seven days before using the mould. Pouring concrete into the mould before it reaches full cure can cause the mould to permanently stretch.