Is Hydrostone Waterproof?

Hydrostone is popular for its hardness and strength, but is it waterproof? Hydrostone is a type of plaster that is used in various applications. If you are planning to create a hydrostone plaster cast that you will display in your garden, you first need to find out if it is waterproof and can withstand rain and other weather conditions. Learn more about hydrostone and if it is waterproof below.

Is hydrostone waterproof? Hydrostone is plaster and plasters are porous materials. Hence, it can still absorb water. Hydrostone can absorb water very slowly. However, the capillary action is strong and can move water at long distances. Moreover, hydrostone castings can weaken and erode due to constant water exposure. Take note that bleach can dissolve hydrogen fast, so avoid using bleach to clean it.

What happens when hydrostone gets wet? Can hydrostone be used outside? How to seal hydrostone? What is the difference between hydrocal and hydrostone? We will discuss these questions and more below. If you are thinking about using hydrostone for making casts for your outdoor decoration, you need to make sure that it can withstand different outdoor conditions. We have also shared tips and ideas on how to seal hydrostone to make it waterproof below.

Is hydrostone waterproof
Image by Wolfgang Eckert from Pixabay

What is Hydrostone?

Hydrostone is a strong and hard gypsum cement. It is commonly used for tooling and for making high-quality art objects and crafts. Hydrostone is also wear-resistant. For more information, you can check out this article titled, what is a hydrostone? We have shared an in-depth discussion as well as tips about the topic that you can refer to.

What Happens When Hydrostone Gets Wet?

If hydrostone gets wet, the water that gets inside can cause it to swell and expand. Prolonged and constant exposure to water can lead to cracks and damages. Take note that hydrostone contains CaSO4 so it is not applicable for vivarium use regardless it is epoxy sealed.

Can Hydrostone be Used Outside?

Hydrostone is not ideal for outdoor use. It is mainly used for making patterns, commercial casts, figurines, and sculptures. Hydrostone is not ideal for making mother or outer molds. Moreover, it is made for strength and casting detail. It is 40 to 50 percent harder as compared to other casting plasters. You can also check out this article that we have shred that discusses how long hydrostone lasts. We have shared an in-depth discussion about the topic.

What is the Difference between Hydrocal and Hydrostone?

The difference between hydrocal and hydrostone is that hydrocal is an excellent material for casting lifecast faces, heads, and torsos from alginate molds. On the other hand, hydrostone is an ideal material for casting statuary and lifecasts such as hands.

How Long Does it Take for Hydrostone to be Completely Dry before You Can Seal it?

How long it takes for hydrostone to completely dry before it can be sealed will have to depend on a few factors like its thickness, and the temperature and humidity of where it is placed to dry. Avoid placing the hydrostone in an oven at the lowest temperature because it can crumble. Moreover, the hydrostone casting should be totally dry before sealing or painting. Otherwise, it can cause crystalization from the salts present in the hydrostone.

How to Seal Hydrostone?

There are many sealing agents that can be purchased on the market and is suitable for sealing hydrostone. You can use a quick-drying lacquer on the casting. Apply it using a brush or spray gun. Two coats of lacquer should be applied to offer the best resistance to moisture. Moreover, the proper procedure should be followed in the application. Make sure to reduce the lacquer by mixing it with the lacquer thinner. This will make it easier for the casting to absorb the first coating.

Next, apply the second coat of lacquer once the first coat has completely dried. When working with plasters like hydrostone, you need to apply the lacquer immediately after it has been set. Apply a heavy coat of lacquer and allow it to dull down. Wipe off the excess by rubbing the casting. You can also sand the casting and apply more coats of lacquer for a high-quality finish.

Take note that gypsum castings need to be sealed to avoid water absorption. This is important especially if you will place the casting outdoors where it will be exposed to various weather conditions. Moreover, porous models, molds, and patterns should be properly sealed.

Hydrostone Plaster Mixing Instructions

Make sure to follow the mixing instructions on the packaging of the hydrostone. Use the proper measurement according to the project that you are working on. When working with any type of plaster product, you need to mix thoroughly for a few minutes. Proper mixing helps avoid water separation that can otherwise damage the surface of the castings with dendritic trails. Proper mixing also improves the working properties of the plaster. Hydrostone that contains hard lumps should be discarded because it means that it has become hydrated and using it can affect the quality of the cast.

Step 1

The first step is to prepare the materials. Use water with temperatures between 70 and 100 degrees Fahrenheit. The differences in slurry hydrostone temperature lead to differences in the setting time. Make sure to keep the water and hydrostone at a stable temperature before using. Slurry with higher temperatures has a shorter setting time. On the other hand, slurry with lower temperatures has a longer setting time. Furthermore, weigh the hydrostone and the water before mixing. The ratio of both materials is critical because it plays an important part in the strength and the density of the cast.

Step 2

The step is to sift the hydrostone into the water. Do this slowly and evenly. Avoid pouring a huge amount of hydrostone into the water because the hydrostone cannot properly soak. Make sure that the hydrostone is completely dispersed in the water before mixing.

Step 3

Next, you need to properly mix the materials. The mixing process of the hydrostone slurry is a vital step to create hydrostone casts with optimal strength, hardness, absorption, as well as other important properties. You can also mix the slurry mechanically to help develop uniform casts with high strengths. When mechanically mixing, you need to do batch and continuous processes. Aside from that, you should use proper blade and bucket dimensions.

Step 4

The next step is to pour the slurry into the mold. Pour the slurry slowly and evenly to avoid air bubbles and to have a uniform and smooth surface. Moreover, agitation or vibration of the filled mold is an important step to prevent air at the surface of the mold. Hydrostone slurry should be carefully poured in the deepest area of the mold. This will make it easier for the slurry to flow across the surface of the mold evenly. Avoid pouring a large amount of slurry directly on the face of the case mold because this can lead to slight densification of the hydrostone mold.

Step 5

Once the slurry has been poured, let it dry. They should dry as quickly and as safely as possible so that they can develop maximum physical properties. Moreover, it should be dried to a constant weight. Make sure that the drying rooms have uniform and rapid air circulation. There should also be equal temperatures throughout the room.

✅ Video – How to Cast Hydrostone 101 Use Plaster Cement Casting

HillBrothersVideo shared the video below on YouTube. It shows how to cast hydrostone. In the video, the process of measuring, mixing, and using hydrostone for casting is shown. Check out the article for the visual demonstration.

Is Hydrostone a Plaster?

Hydrostone is a plaster. It is a gypsum product that is harder as compared to the regular plaster of Paris. Hydrostone also lacks the period of plasticity in its setting curve. As a result, this enables the plaster to be worked directly. Clean the surface of the hydrostone casting as soon as you demold.

Is Gypsum a Cement?

Gypsum is not cement. On the other hand, it is used as a retarding agent of cement. Gypsum is usually used to regulate the setting time of the cement. If gypsum is not mixed into cement, the cement clinker can immediately condense by mixing with water and releasing heat.

What is Hydrostone for?

Hydrostone is primarily used in tooling and making high-quality art object applications. This material is extremely wear-resistant. Hydrostone is also the strongest and hardest gypsum cement available.  You can also check out this article that we have shared that discusses how to use hydrostone. We have shared tips and ideas in using hydrostone that you can refer to.


In this article, we have discussed if hydrostone is waterproof. We have found out that hydrostone is not a waterproof material. Like plaster of Paris, hydrocal, and ultracal, hydrostone is made from gypsum and is a porous material. Hence, it is not ideal for outdoor use. However, there are ways wherein you can seal and waterproof hydrostone. We have also shared tips on how to seal hydrostone to make it waterproof. Thank you for reading!

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