What Can You Spray on Styrofoam to Make It Hard?

hardening styrofoam with hard coat

There are plenty of sprayable materials you can use to harden Styrofoam. Styrofoam is a lightweight material that is easy to CNC and create custom shapes, but it does not provide any strength or rigidity for long-term use. Using a sprayable hardcoat to make the Styrofoam “hard” and durable for use in almost any environment is a necessity. A hardcoat will solve your impact and environmental problems by encapsulating the Styrofoam, leaving only a paintable surface.

These coatings are designed to protect against impacts as well as wind, moisture, sunlight, or anything that may degrade the foam structure. They will not erase any details carved into the foam, as they tend to be applied in multiple coats to build protection.

What is Styrofoam?

Styrofoam is a brand name for closed-cell extruded polystyrene (XPS) foam used in a variety of industries. It’s commonly confused with EPS (expanded polystyrene) foam since people use the brand name to generalize all polystyrene foam. Both types of foam are lightweight yet sturdy and easy to sculpt or carve, making them desired for architectural and theming applications.

Styrene foam is also cost-effective compared to other materials like wood or metal. There’s not a project too big that foam isn’t able to handle since you can carve a single piece or assemble multiple pieces together. Once a three-dimensional object is completely carved, it’s hard-coated to provide a desired finish that is paintable and durable in any environment.

Types of Foam Hardening Materials

The type of method you choose to harden foam will depend on the shape of the structure, the needed finish, budget, and turnaround time. These are typically not the best methods for crafty DIYers who use Mod Podge, PVA glue, adhesives, and paint to protect their smaller projects. If you want your project to last, the following materials are your best bet:

1. Polyurea Coating

Polyurea coatings are durable and flexible coatings with premium properties, allowing you to have great tear strength, tensile strength, and elongation. They are extremely fast setting for quick turnarounds on your foam projects. If you want a foam structure that can be placed indoors or outdoors and withstands abrasion, chemicals, and impact, this is the ideal option. It also provides substantial moisture resistance for use in moisture sensitive environments.

Polyurea coatings must be sprayed through plural component equipment since they cure quickly. Their speed can also make them a little tricky to work with, so special training and equipment are required.

2. Polyurea Hybrid Coating

Polyurea hybrid coatings are made of both polyurea and polyurethane to be strong yet maintain flexibility when applied to foam surfaces. They cure rapidly, so the surface is sandable and paintable within a day. They can also be sprayed over plastic, cardboard, wood, cement, metal, and other prepared surfaces. They are most recommended for indoor and outdoor use, spraying large areas, or use in areas with a lot of traffic. Some coatings can also be fire retardant to meet safety requirements when needed.

These coatings also must be sprayed through plural component spray equipment, which requires training to use. However, they allow you to cover large surfaces quickly compared to other techniques that are more labor-intensive, such as fiberglass.

3. Polyurethane Coating

Polyurethane coatings are also strong, durable, and watertight when applied to various surfaces. They can be applied directly to foam, and unlike polyester resin or other solvent-based materials, they will not melt it. They’ve been used in a variety of foam projects, from architecture to theming and art. Other than foam, they can also be sprayed over fabric, metal, plastic, wood, etc.
They are a bit slower than polyurea hybrid coatings but still cure within hours for sanding and painting the next day. Like polyurea hybrid coatings, they also must be sprayed through plural component sprayers. However, some are offered as a cartridge-based spray system for ease of spraying and low-volume applications. Their chemistry makes them the more cost-effective option compared to hybrids.

4. Epoxy Coating

Like the aforementioned coatings, epoxy coatings form durable, hard-shell finishes over EPS foam. Most of these coatings, however, are not sprayed but brushed or rolled onto the surface. This can make the application process more labor-intensive. Epoxy is also applied in thinner coats, which makes it a bit more fragile than other coatings. Heavy impact could break or crack the coating, but it is a more economical solution.

Epoxy coatings also typically need to be sanded after they cure to create a smoother texture for priming and painting. They are most recommended for limited outdoor use or short-term applications over foam signs, logos, and props.

5. Fiberglass

Fiberglass is a process where once the foam is carved, successive layers of fiberglass mats are laid over the surface and wet out with resin until the desired thickness and strength are reached. This process can be messy and labor-intensive but makes the surface strong and durable as the fiberglass will bond to it.

Using polyester resin with fiberglass mats will melt Styrofoam, so the surface will either need to be covered with something else first, or a different resin will have to be used. Epoxy is the resin typically used in place of polyester. Polyurethane foam can handle either epoxy or polyester resins.

Examples of Hard Coating Foam

  • Theming – Polyurea or polyurethane hard coats can protect foam sculptures, props, sets, and signs for amusement parks, movie sets, or art shows. These coatings must be able to endure a lot since people will probably put their hands on them, children might play on them, and the weather might affect them if they’re placed outside.
  • Construction – Replacing stone architecture with foam is a possibility, but the foam must be hard-coated to withstand the molding process and additional weight.
  • Decorative Accents – Foam hard coats can be used for decorative purposes to mimic crown molding, trim, window shutters, pillars, columns, and more. They can also be used as garden décor for faux boulders, stones, tree bark, and more.
  • Transportation – Depending on the hardness, polyurea hybrid and polyurethane coatings can be made more flexible for seat cushion covers for aircraft, boats, and automobiles. They can also be used as covers for pads on amusement park rides for safety as well as comfort.

VFI Hard Coat Products

VFI manufactures a line of hard coat products for foam applications. These polyurea hybrid and urethane coatings range in hardness from 70 to 95 A and 50 to 75 D, making them durable for indoor and outdoor environments. As a premium option, some of our hard coats will pass Class A fire testing for optimal safety in any application. While most of these are meant to be sprayed, we also offer a brushable hardcoat option.

Contact VFI to learn more about protecting your foam project with a hard coat.