How to Seal Styrofoam for Outdoor Use

There are several ways to seal styrofoam for outdoor use. You should prioritize weather-resistant materials to seal the foam, ensuring the longevity of your piece. Whether it’s rain, sun, impact, or other outdoor factors, styrofoam requires extra protection to maintain its structural integrity, especially if it will be placed in storefronts, public spaces, or outdoor events.

Protection is important, not just for styrofoam, but other foams too. Most foam sculptors in the industry are actually using expanded polystyrene (EPS) over styrofoam (the brand name for extruded polystyrene (XPS)). This is because EPS is made in large blocks, and XPS is made in sheets.

Regardless of which styrene foam you use, both are inexpensive and easy to work with but can become damaged if not protected with a durable hard shell.

Does Foam Last Outside?

Styrene foam is not designed to withstand extended outdoor conditions. One reason foam should be sealed is because it is highly sensitive to UV rays. The plastic material will break down into a discolored powdery substance or become brittle upon long-term exposure. It can fully break down in a few years, which is bad for the environment if not disposed of properly.

In addition to this degradation, excessively heating the material can break down its chemical structure, causing it to leach. Small amounts of styrene will seep out and contaminate surrounding surfaces, which means if your foam piece is within reach of people, it’s not safe. The foam may also lose its thickness during this process.

While it is water resistant, it’s not waterproof. Over time, it will absorb moisture from rain, snow, or spills. Extra water may also be stored causing mold and mildew growth. In addition, there are also a handful of solvents that can melt or break down polystyrene . Impacts from various environmental conditions can also damage uncovered foam. If you want to protect it from damage, it is crucial to apply some sort of coating or sealant.

Options for Sealing Styrofoam

When you seal styrofoam for outdoor use, the material you choose will depend on how long you need the piece protected. While many people use DIY methods because they’re more cost-effective, there are specialized hard coats for styrofoam that lengthen the life of your project.

Polyurethane is the top suggestion for sealing styrofoam. These coatings are two component, semi-rigid materials that are plastic-like and impact resistant when they harden. When applied, they provide the durability and flexibility needed to protect the surface underneath without cracking. Most are applied by high pressure spray equipment, but there are also brushable options, such as VFI-2519 75 D Brushable Hard Coat, for smaller projects. For lower cost spray options, there are even coatings in a cartridge format, such as VFI-6171 70 D Qwik Spray Hard Coat.

Polyurea coatings are a similar alternative but come with premium properties at a lower hardness in comparison. These properties will make the cost slightly higher, but they do offer more in the way of protection in the form of impact and thermocycling. They are usually fast setting and they’re also a great alternative if you work in environments where moisture is an issue. One downside to a polyurea material is that it is normally harder to sand, because they tend to be softer than a urethane hardcoat.

Epoxy is a coating with similar characteristics to polyurethane and polyurea. It acts as a protective barrier for any type of foam but is applied in several thinner coats. Epoxy is also mostly brush-applied or roll-applied, which makes it time-consuming to work with, so it’s typically used for smaller projects. If you plan to sand the material, particles get into the air and can create toxic dust, so an approved respirator should always be used. The main benefit is the ability to work on large projects without the need for a full spray booth.

Fiberglass has a more time-intensive application process, but it is an effective way to protect an outdoor foam piece. Layers of the material are placed over the foam with intermittent applications of epoxy or polyester resin that will impregnate the fiberglass fabric/fibers. These layers are applied until the desired thickness is achieved providing strong impact resistance that is reinforced by layers of fabric. Resins that are not a full 100% solids will melt the foam, so you must be careful in choosing the right one.

Benefits of Sealing Styrofoam for Outdoor Use

  • Protection – There are several things a hard coating can protect outdoor styrofoam projects from. The coatings resist moisture and water damage, UV radiation (when a compatible paint or topcoat is applied), and other weather conditions. They also resist impact and abrasion from people sitting on, climbing up, or touching these structures. Some coatings can also provide fire retardant characteristics to meet fire safety requirements where necessary.
  • Long-term durability – Because a coating can offer incredible protection, it keeps your foam piece in good condition, especially outside. If you were to just paint your foam piece and call it a day, there’s no doubt that it would deteriorate quickly and not look very smooth. When your project is resistant to external factors, it lasts longer and saves you money in the long run.
  • Versatility – There’s no limit to the shapes and projects you can use a hard coat for. Because spray coatings are the most used and recommended application option, the coating gets into all the curves and crevices on a piece. These coatings adhere well to all types of foams, so you gain protection no matter what material you use.
  • Easy to apply & work with – Most hard coatings are relatively easy to work with, as long as you have experience with high pressure spray equipment. Most formulas also come in 1:1 mix ratios for ease of setup. If spraying is not your thing, brush and roller applications are also available. After application, coatings are easy to post work. Fast setting coatings can leave a smooth surface perfect for top coating or painting, especially when spray applied.

Outdoor Applications for Sealed Styrofoam

Due to the adaptability of sealed foam, a handful of outdoor applications benefit by using these materials. These materials are used to fabricate signs for outdoor store displays, theme park rides, restaurants, and more to enhance brand recognition and draw customers in. Nothing elevates an aesthetic like a custom sign that stands out from less creative materials.

Companies may also use foam to create large-scale 3D figures or characters to attract visitors and create magical environments. They can be seen at outdoor festivals, concerts, parks, gardens, etc. With a hard coat, your creations will be protected for years.

Sometimes, these materials are used for realistic outdoor hardscaping, such as stones, edging, boulders, rocks, and more. Foam hardscape objects can be more cost-effective since natural materials are more expensive and difficult to install. Heavy machinery is typically not required when installing hard-coated foam pieces, reducing labor costs.

Outdoor amusement parks rely heavily on custom props to immerse customers and enhance their experience. Hardened foam can be used as themed elements in ride queues, in the rides themselves, or throughout the park. The protection these hard coats offer keeps your projects in good shape for a long time.

Contact VFI today to see if you could benefit from using a hard coat on your next foam project.