How to Make Styrofoam Hard

White EPS Styrofoam can be hard coated with liquid polyurethane.
White EPS Styrofoam can be hard coated with liquid polyurethane. (Image: sirastock/iStock/Getty Images)

Keep your Styrofoam props, cosplay outfits and other projects protected with a hard-surface coating. You don't need to rely on messy fiberglass resin to harden your Styrofoam. Simply use a two-part liquid polyurethane hard-coat system.

Things You'll Need

  • Styrofoam liquid primer
  • Polyurethane hard-coating system
  • Paint tray
  • Paint rollers
  • Paint brush
  • Paint mixing spatula
  • Plastic containers
  • Spray gun
  • Respirator
  • Gloves
  • Drop cloth

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Priming the Styrofoam

Spread out the drop cloths on a clean workspace in a warm, well-ventilated area. Open the can of primer. Mix well within the can using the paint mixing spatula.

Pour a small amount of Styrofoam liquid primer into the paint tray. Roll the paint roller in the tray to coat it with primer.

Roll the primer onto the surface of the Styrofoam. You can to apply an even layer of primer to seal the surface. Try to push the primer into the small crevices between the Styrofoam bubbles. Allow the primed Styrofoam to dry overnight.

Hard Coating the Styrofoam

Spread out your drop cloth in a clean, well-ventilated workspace. Put on a pair of protective gloves. If you plan on spraying the liquid polyurethane onto the Styrofoam get out your respirator and set up your spray hopper. Otherwise, you can just use a paint brush and paint tray to brush on the liquid polyurethane.

Mix the liquid polyurethane together. Most polyurethane hard-coating systems come in two parts – component A and component B – that need to be mixed prior to application. Stir component B with a paint spatula inside the bucket prior to mixing. Pour equal amounts of component A and component B into a plastic container. Stir until well mixed with the plastic spatula, about one to two minutes. Pour some of the liquid polyurethane into a paint tray.

Load your paint brush with the polyurethane. Brush the polyurethane onto the Styrofoam surface. You want to completely cover the surface with one even coat of the polyurethane. Allow the first layer to dry for about 40 minutes to one hour.

Brush between two and four total coats of liquid polyurethane onto the Styrofoam surface. Allow up to one hour of drying time between each coat of polyurethane. You will need only two coats for a small Styrofoam block, and up to four coats for a large project.

Tips & Warnings

  • Wear protective gloves to keep the liquid polyurethane from touching your skin.
  • You can add color pigment to the liquid polyurethane before applying it to create pastel colors, or you can paint the Styrofoam after the hard coat has dried.
  • If the polyurethane bubbles or wrinkles, it is likely that it wasn't completely dry between coats or the coats were too thick.
  • If the polyurethane remains sticky, it was probably mixed unevenly or you didn't use a proper 1:1 ratio.

References

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