Why Is My Concrete Casting Shiny?

Decorative concrete elements are used to enhance the architecture of residential and commercial structures with various colors, textures, and patterns. Their most desirable trait is the ability to mimic other natural building materials at a lower cost.

However, issues can occur in the manufacturing process that will affect the casting’s final finish. These issues can make the casting appear shiny rather than matte, like natural stone, brick, and wood. The shiny appearance will typically occur from using too much mold release or bad pigments.

Reasons Your Concrete Finish is Shiny/Glossy

1. Overapplying Mold Release or Sealer

A shiny casting typically occurs from mistakes made in the initial process of making the concrete mold. One of the most popular materials for making these molds is liquid urethane rubber. When trying to make a mold through additive or subtractive manufacturing, it will always appear to be man-made or artificial. Urethane rubber will avoid the artificial look by copying the natural profile of a stone and provide a matte finish.

Before using the rubber, you have to prepare the molding surface. It’s how you prepare the surface that can change the final finish of your mold and future castings.

Situation 1: The Master

In the process of preparing your mold box or form, you have to apply a release agent. You might also have to apply a sealing agent directly on porous masters. Both these materials will help prevent the rubber and molding surface/master from sticking together. However, the amount of release and sealer you use can affect the finish of the mold. If used in excess, these materials will cause shininess on the mold.

For sealing agents, VFI typically recommends using a combination of 80% mineral spirits to 20% petroleum jelly by weight on porous surfaces. If your mixture is too heavy on the petroleum jelly and you’re overapplying it, it can increase the shininess of your master.

On mold boxes and masters with intricate details and low points, overapplication can lead to pools or puddles of mold release and sealer. This will create a residue on your molding surface that will be picked up by the rubber as it cures, resulting in a glossy finish. Not allowing the release or sealer to dry before pouring the rubber can also result in a poor surface finish.

After demolding, you’ll notice a shine in certain spots on the interior surface of the mold. This occurs because the urethane rubber is able to capture extreme detail and texture. This ability is why it’s such a desirable material for concrete molds, but it can also make it tricky to get the finish you desire. If you were to cast concrete into that mold, every casting that comes out of the mold will have distinct areas with a glossy appearance.

Situation 2: The Mold

Maybe your urethane rubber mold wasn’t shiny after you demolded it, but there’s still a chance you could make shiny castings. The mold will still need to be released to prevent the concrete casting from sticking to it. If you use too much release on the mold or don’t allow it to dry, it can also create a shiny finish when you cast the concrete. Sometimes, it can even cause pinholes and other defects in the concrete. If you don’t use release during this step, the mold will bond to the wet concrete.

2. Bad Pigments Were Used

Decorative concrete manufacturers typically color concrete with powder, liquid, or granular pigments. Pigments transform the concrete from a standard gray color into whatever color you need to replicate natural building materials. Pigments will either be added directly to the concrete mix or applied to the surface of the mold. Using pigments that are incompatible with or have surpassed their shelf life can cause the concrete to appear shiny.

Solutions

Applying a release agent is a delicate balance in both the molding and casting process. Avoid using too much sealer or release in either step. If pools of these materials are not wiped away and allowed to dry before casting, they can cause permanent shine and glossiness, discoloration, and even bubbles. If you remove the excess ahead of time, it shouldn’t impact the finish.

Always check the manufacturer’s instructions when using new materials. A good rule of thumb when using release agents is to apply 2-3 coats of the material in thin films. Wait for the release to dry before applying another coat. The same can be said when using a sealing agent. This will help eliminate pools of material.

Be aware that you will have to apply release to the mold after each casting as the release will wear away. This should help you determine how much release is needed for effective demolding and to produce castings without a shiny surface. Before applying more release, you’ll also want to clean the mold to get rid of previous release or concrete residue.

Because most decorative concrete projects use pigments to color the concrete, check the expiration date before use. Otherwise, it could result in a poor surface finish.

Contact VFI if you’re still experiencing issues with your castings appearing shiny, and we will do our best to provide potential solutions.