One technique for getting a rubbed concrete finish is called "sacking" because the traditional way to do it is to use an old burlap sack. It isn't the only way to finish concrete, and it isn't necessarily the best. An alternative method called "parging" makes use of stiff-bristle brushes to apply the concrete finish coat and smooth it down.
Sacking a new concrete wall serves two purposes: It makes the finish very smooth, and it makes the color uniform. When you pour new concrete, small holes, crevices, and other defects are inevitable and need to be filled. Oftentimes, the fill material and the original concrete have slightly different colors, and sacking is a remedy for that.
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Two Ways to Get a Sack Rub Finish
If you're going to use the traditional method for a sack rub finish that employs a burlap sack, there are two ways to do it, and the best choice depends on the condition of the concrete. If the concrete is already quite smooth and just needs a final surface coat, much like a tabletop needs a lacquer coat, you can wet the wall, fill the sack with a mixture of sand and Portland cement, and rub that over the wet wall. This is called dry sacking. Burlap is porous enough to allow the mixture through, and the material is coarse enough to rub it down.
If the wall has a significant number of defects, it's often better to apply a thicker finish coat than is possible with a sack. In this case, you need to wet sack. You can do this with a trowel or a stiff-bristle brush, and you can still use the burlap sack. You just crumple it into your hand and use it for rubbing.
How to Dry Sack a Wall
Before dry sacking a concrete wall, you first need to wet down the concrete with a mixture of water and concrete bonding adhesive and let it sit until the surface moisture has evaporated but the concrete is still wet. Before you do that, you'll want to grind down bumps with a grinding pad and wipe the wall clean of dust. Wet the concrete by dipping a large-bristle brush into the water/bonder mixture and basically painting the wall with it.
Next, combine equal parts of Portland cement and fine sand in a bucket and stir to completely mix them into a uniform powder. Put some powder in a burlap sack and rub the sack over the wet wall. The concrete/sand mixture will form a thin film on the wall that you can smooth by rubbing repeatedly with the sack.
How to Wet Sack a Wall
When you need to wet sack a wall, it's because there are too many large defects to hide with the dry sack method. Before sacking, you need to grind off protrusions and fill deep holes and crevices with an appropriate patching compound. Mortar often works for filling, but you can also use a commercial patching product. Wet the wall with water mixed with binder before applying the mortar.
The mortar you use for a sack and patch concrete finish is the same mixture of Portland cement and sand, but it should be mixed with water to a paintable consistency. You can spread it on the wall using a trowel as if you were skim coating, but a large-bristle brush is easier. Let the finish coat stiffen slightly and then rub it lightly with a piece of burlap to smooth it. Rub in a uniform direction to avoid streaking.