How to Make Wood-Textured Concrete Stools

Concrete Log Stools

The following tutorial details the process of making a polyurethane rubber mold of a log that will be used to cast concrete stools.

A list of supplies and tools used for this project are provided at the end.

The Process:

Step 1: Prepare the Original Model
Step 2: Construct a Containment Area
Step 3: Measure, Mix & Pour FormRub 35 Liquid Rubber
Step 4: Demold & Cut the Mold
Step 5: Cast Concrete


Step 1: Prepare the Original Model

Preparation of the original model is a very important step in the mold making process. Improper preparation can lead to the rubber bonding to the model or improper cure of the rubber.

For porous models that are brought in from the outdoors, it’s important to allow them to fully dry as moisture can affect the cure of polyurethane rubbers.

Porous models, such as this log (below), must be thoroughly sealed to prevent rubber from penetrating the pores and becoming stuck. Many sealers can be purchased through local home improvement stores while others are available directly from Polytek.

How to make a mold of a log

In some cases, a couple of layers of primer spray paint will be enough to seal the model; however, this log is extremely textured and porous, so we decide to paint on a layer of paint by hand. Brushing on the paint allowed us to “push it” into the texture better and seal the surface more thoroughly.

Apply Primer Paint to Log

Once the model is thoroughly painted, a release agent must be applied. Again, since this log is extremely prous and the bark texture is very deep, we decided to use PolyCoat. PolyCoat is a silicone-based, semi-permanent sealer and release agent.

PolyCoat Sealer and Release Agent

The log is turned onto its side to help prevent runoff and allow the PolyCoat to fully penetrate the irregular surface. In total, four layers of PolyCoat are applied.

PolyCoat should be allowed to cure in between application of each layer and the final layer should be allowed to cure before application of any liquid rubber.

Sealer & Release Agent for Porous Models

When the PolyCoat has cured, a large crack in the bottom of the log is filled with warmed plasticine clay to help prevent rubber from seeping into it (the side of the log pictured below will be the bottom of the finished concrete stool.) A clay “snake” (i.e., clay that has been rolled out into a long, thin strip) is also placed around the outside edge of the bottom of the log to act as a gasket when the log is mounted onto the baseboard.

Prepare the Model for Mold Making


Step 2: Construct a Containment Area (a.k.a. Mold “Box”)

The log is about 12″ in diameter, so we manufacture a plywood baseboard with a 14″-diameter recess. A 14″-diameter cardboard concrete tube (e.g., Quik-Tube® or SonoTube®) will fit snugly into the recess and will act as the containment area. The recess is only about 1/2″ deep, but it will help to keep the tube stable when the mold rubber is poured.

A 14″-diameter tube is selected so that the mold walls will be about 1″ thick.

Baseboard with 14-Inch Recess

The baseboard is placed onto the bottom of the stool and secured with screws. It is important to make sure that the work surface and log are level throughout this process.

Mold Making Tutorial

Vent holes are made in the baseboard so that air in the model may escape. Without vent holes, this air may become trapped in the rubber.

Drill Vent Holes in Baseboard

Screws and Vents in Baseboard

The edges of the log are sealed with plasticine clay to help prevent rubber from seeping beneath the log. The mold rubber will pick up the clay texture, so the clay is flattened/smoothed out as best as possible.

Seal Edges of the Log

Clay is also placed around the edge of the recess to help prevent rubber from leaking outside of the containment tube.

Clay Gasket

A cut is made down one side of the tube; this will make the demolding process easier, but will also weaken the tube.

Cut 14-Inch Diamater SonoTube

Before placing the tube around the log, we tape the inside and outside of the cut.

Tape the Cut in the Tube

Vaseline is then applied to the entire inside of the tube with a dry brush. The exterior side of the bottom of the tube is also coated just in case rubber leaks outside of the tube.

Coat Inside of Tube with Vaseline

Tape is wrapped around the tube and then the tube is placed snugly into the recess. Mold straps are used to secure the tube and baseboard together.

Place Tube Around Log and Secure

Secure Mold Straps and Tape


Step 3: Measure, Mix & Pour FormRub 35 Liquid Rubber

FormRub 35 Liquid Rubber is a two-part (Part A & Part B) polyurethane system that cures to a Shore A35 hardness.

Basic Specifications
Mix Ratio: 1A:1B
Initial Mixed Viscosity: 600 cP
Pour Time: 15 minutes
Demold Time: 16 hours
Cured Color: Turquoise

FormRub 35 Liquid Rubber

To determine how much rubber is needed to make this mold, we do the following calculation:

  • Determine the volume (in³) of the containment area.
  • Estimate the volume of the log and subtract it from the volume of the containment area.
  • Take that result and divide by the specific volume of the mold rubber (the specific volume of FormRub 35 is 27.2 in³/lb).
  • This final result is the amount of rubber, in pounds, needed to complete the mold.

This log mold requires approximately 65 lb of FormRub 35 Liquid Rubber.

FormRub Series rubbers can be mixed by weight or volume, but we highly recommend weighing on a digital scale.

Because these mixing pails only hold 40 lb of material, we mix three separate batches of rubber.

Part B is added to the mixing pail first because it is lower in viscosity and less likely to cling to the sides.

Measure Part B - FormRub 35

Part A is then weighed into the bucket.

Meaure Part A - FormRub 35

The components are thoroughly mixed. It’s important to scrape the sides and bottom of the bucket multiple times during the mixing process.

Keep the pour time of the product in mind when mixing. FormRub 35 has a pour time of 15 minutes, but this time varies from product to product.

A Poly Paddle, used below, is a good tool for hand mixing.

Thoroughly Mix FormRub 35

The rubber is poured into one side of the mold. We continue to pour the rubber into the same spot and allow it to rise.

Pour Mold Rubber into Tube

When working with multiple batches, pour the next batch as soon after pouring the previous batch as possible.

FormRub 35 Mold Rubber

Generally, mold rubber should be poured to at least 1/2″ above the top of the model. For this mold, we pour the rubber about 1″ over the top of the log.

Allow FormRub 35 to Cure

FormRub 35 can be demolded after 16 hours at room temperature.


Step 4: Demold & Cut the Mold

The tape that covers the cut in the tube is cut and then the tube is removed from the mold.

Demold FormRub 35

Remove Tube from Rubber Mold

The screws are removed from the baseboard and the edges of the mold are loosened with a flat tool (e.g., putty knife). The mold is then turned over (this may require two people) so that the bottom of the log is exposed.

Remove Log from Baseboard

Remove Mold from Baseboard

Flip Mold Over

A cut will be made down one side of the mold to remove the log and subsequent castings. A sharp scalpel or blade is a good tool for cutting soft, polyurethane rubbers.

It can be helpful to have a second person pry open the cut; this gives a better view of where the model is. The location of the cut of this mold is not incredibly important because a seam line in the final casting is well-disguised in the deep textures of original model. Seam lines can be more noticeable depending on the shape and texture and original model, so cutting the mold along an existing line in the model is important in those instances.

Cut Rubber Mold with Scalpel

Cutting in a zigzag pattern (compared to a straight cut) is helpful for realignment of the mold when casting.

Cut in ZigZag Pattern

If rubber has seeped into any deep knots in the log, it can usually be cut away easily with a scalpel.

Make Cut in Rubber Mold

While one person holds the mold apart, a second person removes the log.

Polytek - Rubber Mold of a Detailed Log

FormRub 35 Rubber Mold - Pollytek


Step 5: Cast Concrete

To cast concrete, the tube is placed back around the mold and secured with a mold strap; this will help to prevent deformation of the mold from the weight of the concrete. It is situated so that the cut in the tube is on the opposite side of the location of the cut in the rubber mold. This will provide more stability during the casting process.

Realign Mold and Tube for Casting

The mold is coated with Pol-Ease® 2650 Release Agent. Pol-Ease 2650 is a silicone-free, oil-based release agent that does not require any drying time before casting concrete. There are a number of other concrete release agents that are suitable for use with polyurethane mold rubbers.

Concrete Release - Pol-Ease 2650 Release Agent

The concrete is mixed and poured into the mold.

Mix Concrete

To reduce the weight of the finished stool, a small amount of concrete is poured into the mold and then a 6″-diameter foam plug is inserted.

Pour Concrete into Mold

Foam Insert in Concrete Stool

A weighted pail is placed on top of the foam insert so it doesn’t float and then the remainder of the concrete is poured into the mold.

Pour Remainder of Concrete

Once cured, the concrete is removed from the mold.

Demold Concrete

Open Rubber Mold

Remove Concrete from Rubber Mold

Excess foam is trimmed from the insert.

Trim Foam Insert

Concrete Casting - Mold Making Rubber

If desired, the concrete can be stained.

Concrete Log Stools

Concrete Casting Tutorial

Wood-Textured Concrete Stools

When properly taken care of, this mold can be used to cast hundreds of concrete stools.

Supplies and tools used for this project: 

Would you like to speak with Polytek Technical Support about your concrete casting project?

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