Styrofoam is a versatile material that can be helpful in everything from construction projects to basic crafts. It has many advantages, including that it is light and easy to work with, relatively inexpensive, and can serve multiple purposes.
However, waterproofing is not one of Styrofoam’s better qualities. Unfortunately, this material is not resistant to water or moisture.
Styrofoam’s permeability can pose a problem in construction, where you want material to be resistant to vapors to protect the structural integrity of your home. It can also mess up a craft project.
Although Styrofoam is not inherently waterproof, there are still ways to boost its strength in this regard. Here are a few methods that you can use to waterproof Styrofoam.
How Waterproof Is Styrofoam?
At first glance, it seems as if Styrofoam should be waterproof. It is made of polystyrene plastic, a chemical compound that is waterproof (although it is not resistant to water vapor, as anyone who has carried plastic takeout containers and then felt condensation forming will know).
Styrofoam also has a tight, closed-cell structure that does not seem as if it would allow water to leak through. In fact, if you tried spritzing water on Styrofoam, it would not flow to the other side, showing that the material provides some barrier to the water.
However, Styrofoam can get waterlogged over time. The gaps between the closely-knit foam cells are not large enough to let water pass through from one side of the material to the other, but they are big enough to let water collect in the gaps.
To be classified as completely waterproof, a material cannot allow water to pass through or absorb any moisture. Although Styrofoam may not leak, even waterlogged Styrofoam can still cause damage. Waterlogging ensures that Styrofoam loses its insulating ability.
Waterlogging can also warp Styrofoam’s form and impact its structural integrity. Too much water, and the Styrofoam will start to contract and break.
How to Waterproof Styrofoam
Thanks to some of its water-resistant capabilities, Styrofoam is fairly easy to waterproof. All that you need to do is ensure that the gaps between the foam cells are covered.
As Styrofoam only becomes waterlogged through direct contact with liquid, not by absorbing vapor from the air, your job will already be easier.
Here are a few easy ways to waterproof Styrofoam.
1 – Use Epoxy Resin
Epoxy resin is a staple product for crafters, boaters, interior designers, and anyone who works with materials that need waterproofing and protection.
Epoxy resin is a type of chemical or polymer that is very strong and reactive. It is highly resistant to all kinds of environmental impacts, including water.
Although epoxy resin is strong enough that it is used on airplane parts, it is also easy enough to use that many at-home crafters use it. Just be sure to put down newspaper or old cloth to protect your surfaces as the application and drying process can be very messy.
However, when you are buying epoxy resin, look out for coatings that are suitable for Styrofoam. Some epoxies are too aggressive and will melt the foam.
2 – Use a Polyurethane Coating
If you are using Styrofoam for industrial or construction purposes, then you will need a stronger, more industrial solution for waterlogging Styrofoam.
Polyurethane coating is a protective coating made out of chemical polymers. It bonds to surfaces such as Styrofoam and protects them against wear and tear and the elements. You can get polyurethane coatings such as DuraSpray online or in specialty stores.
If you have a large surface of Styrofoam to coat, then you can get a larger bottle of polyurethane coating, which often comes in several gallons with a spray top for easy application. You can also apply it with a paintbrush.
Some polyurethane coatings come in two parts. The manufacturer will usually label them component A and component B and give you instructions on the proper ratios. You can use a paint stick to mix the two together.
Polyurethane coatings have several advantages over similar coating materials. The first is that they are much gentler on the base material than harsher chemical compounds and they will not melt or damage the Styrofoam.
Polyurethane coatings are also more flexible, while epoxy resin often dries hard and shiny. However, polyurethane still hardens Styrofoam somewhat, so expect the texture to change.
Polyurethane coating also dries within a day, while you often need to let epoxy coatings rest for several days before you can move or use the Styrofoam.
For best results, apply a Styrofoam-specific primer before going over it with the coating.
3 – Polyurethane Resin
If you are using Styrofoam for crafts and you don’t want to invest in industrial-strength polyurethane coating, polyurethane resin is a similar material that will also help waterproof and protect Styrofoam.
Polyurethane resin usually comes with small jars that you can apply with a paintbrush. It is so durable that many theater professionals use it when making stage sets, props, and mock-ups.
Just be warned that polyurethane resin tends to dry very shiny so it will affect the texture of your final piece.
4 – Use a Sealant
Using a waterproof sealant for Styrofoam is one of the best ways to protect this material, whether you are using it for artistic or construction purposes. You can get sealants such as FoamCoat online, in hardware stores, or even in some arts and crafts stores.
The application process will depend on the sealant. Some come as aerosol sealants, but most come in a bucket and you apply them like paint.
To seal the Styrofoam, clean the surface and make sure that it is free of dust and debris. Then, use a paint brush or roller to go over the surface. For best results, apply several coats.
Finally, wait at least 24 hours or according to the package instructions for the sealant to dry. Once it does, your Styrofoam should be waterproof.
Most sealants are easily compatible with paint and textural work, so apply the sealant first and then get started on your other work. Sealants are also highly durable to wear and tear over time, which is why this is another tool that is popular in the theatrical business to make props.
Can You Use Aerosols on Styrofoam?
Many sealants and epoxies come in an aerosol format. There are many advantages to using an aerosol to apply sealant. It is much easier to apply, often creates less of a mess, and you don’t need to get other equipment such as a paint brush or roller.
However, some chemicals used to create aerosol sprays are actually highly dangerous for Styrofoam. Depending on the aerosol, the chemicals could dissolve parts of the Styrofoam, leaving you with no foundation to work with.
It is better to play it safe and use a sealant that you can apply with a paint brush or roller. This ensures that you get all of the benefits of a waterproof coating without accidentally damaging the Styrofoam.
If you only have an aerosol on hand and you desperately need to seal some Styrofoam, there is a way to minimize the damage to the foam. Spray the aerosol into a cap or small container and wait a few minutes for the aerosol chemicals to dissipate. Then, apply it to the Styrofoam with a brush.
If you are using the latter method, just be careful that you are not making something that requires structural integrity as you could still damage the Styrofoam. If you can, try to have extra foam on hand.
Why Is My Styrofoam Hardening?
Styrofoam is popular for its soft, flexible surface and absorbent properties. However, you may find that after applying your sealant or resin that the Styrofoam is hard to the touch.
This is pretty hard to avoid because to make the Styrofoam waterproof, a coating needs to cover up all of the gaps in the foam-cell structure. Naturally, this means that the structure will be less flexible.
Your Styrofoam may not be as soft and fluffy to the touch, but adding a waterproof coating will ensure that it is more durable. If it is rigid to the touch, that means that the protective surface is working.
Many prop designers, theater geeks, and cosplayers use waterproof coatings over Styrofoam to preserve their creations for several years to come. If your Styrofoam is hard, that means that it is more immune to all kinds of environmental damage than it was in its original form.
Styrofoam is a versatile material that is the backbone of many personal crafts, theatrical productions, and even construction. It is long-lasting, durable, and water-resistant, but not waterproof. Its molecular structure is just porous enough to absorb water. If it is exposed to too much water, Styrofoam can become waterlogged.
Luckily, you can easily make Styrofoam waterproof by using several different materials, including resin, polyurethane coating, and sealant.
Just be sure that the coating you are using is Styrofoam-safe before you start applying it to avoid ruining your project. Some coatings have harsh chemicals that cause Styrofoam to break and even dissolve.
I have a bachelor’s degree in construction engineering. When I’m not constructing or remodeling X-Ray Rooms, Cardiovascular Labs, and Pharmacies…I’m at home with my wife, two daughters and a dog. Outside of family, I love grilling and barbequing on my Big Green Egg and working on projects around the house.