DiyProject

How to Sew a Box Cushion With Extra-Firm Flotation Foam

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Welcome to another Sailrite® DIY tutorial! If you’ve ever worked with closed-cell or flotation foam, you know it’s a completely different animal than standard upholstery foam. Because it doesn’t compress, your usual sewing rules don't quite apply. In this video, Eric Grant walks you through the specialized techniques needed to achieve a professional, bubble-free finish on cushions that are built to last in the toughest marine or outdoor environments.
Products:
  • Sattler Fabrics
  • Blade Foam Saw
  • CushionRite Flotation Foam
  • CushionRite Spray
  • Foam Batting
  • Measuring Tape
  • Zipper Chain
  • Precision Cutting Foot
  • Hotknife
  • Fabric Notching Tool
  • Sewing Machine
  • Clear Acrylic Ruler
  • Chalk Pen
  • Basting Tape
  • Canvas Patterning Ruler
  • Thread Snips
  • Locking Zipper Pull
  • Eversoft Piping
  • Sewing Gauge Ruler
  • Magnetic Sewing Guide
  • Fabric Edge Match Pliers

Why Use Flotation Foam?

Closed-cell flotation foam is the gold standard for boat cockpit cushions and heavy-duty outdoor seating. It’s extra firm, water-resistant, and—as the name suggests—it floats! However, because this foam is so dense, a standard "snug" fabric cover would cause the cushion to bow or bubble out.

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"Bowing" may occur using a standard cushion making approach

In this project, we show you how to adjust your measurements and assembly to accommodate these unique properties. Whether you’re sprucing up your patio, adding durable comfort to your boat, or creating custom seating for an RV, mastering this box cushion style is a game-changer for your DIY repertoire.

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Key Techniques You’ll Learn

This video is packed with "tricks of the trade" that make sewing heavy-duty materials much easier. You’ll see exactly how to:

  • Glue and Cut Dense Foam: We use the Sailrite® Blade Foam Saw to get perfectly square 90-degree edges on 3-inch thick foam.

  • Adjust for Zero Compression: Learn why we cut the fabric plates a quarter-inch larger than the foam itself to prevent that dreaded "edge curling."

  • Create Zipper Pockets: We demonstrate how to sew a sleek zipper plaque using SewReady™ Seamstick Basting Tape and a Lenzip® #5 Molded Tooth Zipper, including a clever "pocket" to hide the slider.

The Right Tools for the Job

Working with marine-grade materials, such as the Sattler® Marine Grade Fabric shown in this video, requires a machine capable of handling bulk. The Sailrite® Ultrafeed® Sewing Machine quickly handles multiple layers of acrylic fabric and zipper tape.

To keep your edges from fraying and to ensure precision, we also utilize the Sailrite® Edge Hot Knife. Using the cordless version with the precision cutting foot allows you to cut directly on your tabletop without a cutting glass, sealing the fabric edges as you go.

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The Sailrite Edge Hotknife seals fabric edges as you cut

Take Your Project Further

If you’re looking to add even more flair, this box cushion can easily be customized with decorative piping. While we kept this version clean and modern, you can follow our other tutorials in this series to learn how to add vinyl or fabric-covered welting for a high-end look.

Ready to build a cushion that can stand up to the elements?

Materials List
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