How to Paint Styrofoam and Make it Hard: The Complete Guide

Styrofoam is a brand name for expanded polystyrene, a petroleum-based plastic made from styrene monomers. Its generic name is EPS foam. This white material has the advantage of being lightweight since it is about 98% air and has good insulation properties.

How to paint styrofoam can be tricky. This is because many paints and coatings contain acetone, or solvents such as benzene, toluene, and tetrahydrofuran. They all dissolve polystyrene, causing it to lose its structure. When it dissolves, all the air escapes, reducing the EPS foam to almost nothing.

This article will look at the ways to cope with hardening styrofoam and painting it, both as a craft material and on a larger scale.

Uses of EPS Foam

EPS foam has many uses as food packaging and in the construction industry. Aside from its good thermal insulation, it has a high impact resistance, which is useful for protective packaging. Like other plastics, EPS foam can be molded, making it versatile.

In the construction industry, its uses include, but are not confined to:

  • Structural Insulated Panels (SIPs)

  • Thermal insulation for walls, floors, and roofs

  • Cavity wall insulation

  • Sound insulation

EPS foam is a very effective sound insulator. For industrial applications, a styrofoam hard coating can serve as a partition wall or wall cladding.

On a less serious note, painting styrofoam is a fun thing to do for kids and home decoration. As a craft, how to paint styrofoam and harden it is quite simple.

How to Paint Styrofoam for Craft Projects

Acrylic paint will not dissolve the styrofoam and sticks to it well. Acrylics and tempera—also a possibility—are stocked by local and online craft stores. There is a wide range of colors, and they usually sell such paints in small quantities of around 2 ounces.

Craft stores stock several branded regular craft sealants. You apply the sealant first to the styrofoam object you wish to paint with either a foam brush or a paintbrush. Applying a small amount at a time to get an even finish on the surface is best.

We must leave the sealant to dry before we can paint the styrofoam object. A foam sponge, foam brush, or soft-bristle paintbrush are good for painting. Allow the first coat of paint to dry completely (this takes about 20 minutes) before applying a second coat if required.

For industrial and commercial purposes, however, a completely different approach is needed. The EPS foam requires a hard coating so that paint can be freely applied on top of it.

Rigid Hard Coating for EPS Foam

EPS foam coating is manufactured industrially. There is a solution in the form of a two-component, semi-rigid, 100% solids, polyurethane polyurea hard coating, especially for EPS foams. It is applied as a spray, for ease of reaching all parts of non-uniform surfaces to which it is typically applied.

It is used for the fabrication of signs, decorative themes, and displays, and concrete molds cut from EPS foam. Applications exist that no longer require high-pressure spraying equipment to apply the hard coating.

Once the hard coating is completely cured, the surface of the hardened styrofoam can be sanded or machined. EPS foam with a rigid hard coating is an effective substitute for fiberglass in some contexts. It can also be painted with either water- or solvent-based paints.

You Need a Primer for The Rigid Hard Coating

Like most coatings, you need a primer before applying the coating.

In the primer’s case for the rigid hard coating mentioned above, the water-based acrylic latex rubber coating is formulated to dry fast. It also adheres very well to cut or carved EPS foam surfaces.

It seals the surface of carved or molded EPS foam before we apply the rigid hard coating. The primer limits air bubbles in the hard coating. It also restricts air bubbles from escaping from the EPS foam surfaces when the rigid hard coating is applied.

Do You Need Both the Primer and the Rigid Hard Coating?

The primer is only required if you want a smooth surface. In all other cases, using the rigid hard coating on its own will do the job satisfactorily.

The advantage of using a primer is that it has excellent flexibility over a range of extreme temperatures. It also performs well at sub-zero temperatures and has high tensile strength.

It is always a good idea to use the correct preparations for the material you’re working with. How to paint styrofoam has many other options, but if you want the effect to be a lasting one with a professional touch, then you need to use professional products.

Long-lasting Solutions

When using these professional products you should follow the manufacturer’s instructions. For example, it is important to apply the primer at a minimum thickness of 7 mm wet, or 6 mm dry. That way you will derive maximum benefit from its tear strength and tensile strength properties.

The primer which is also white, like styrofoam, needs sufficient time to dry before spraying on the rigid hard coating. The cure time for the primer is around 24 hours at an ambient temperature of 72°F. High humidity conditions, cooler temperatures, and a thicker film of primer need longer to cure.

The rigid hard coating needs between 20 and 30 minutes to set. Once you have done that, you can apply paint suitable for bonding with plastic to the surface as required.

The primer and the rigid hard coating are both classified as Class 55—non-hazardous solid waste—since they contain no volatile organic compounds (VOCs). However, you still need to dispose of empty containers in compliance with federal, state, and local regulations.

Technical Solutions for Greater Creativity

Once you have figured out how to paint styrofoam and make it hard, there is no limit to your creativity. Colorful interior decorating for shops, restaurants, children’s play centers, promotional points of sale and even outdoor signage are all possible.

Browse around our magazine for other decor ideas, and much, much more.

About RJ Frometa

Head Honcho, Editor in Chief and writer here on VENTS. I don't like walking on the beach, but I love playing the guitar and geeking out about music. I am also a movie maniac and 6 hours sleeper.

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