Interfacing can be a great tool to add extra structure to an accessory when the outer fabric alone doesn’t have enough body. For instance, it could be used to help a pouch maintain its shape, reinforce a zipper area, give more stability to a flap, or prevent a fabric organizer from collapsing. The key is to choose a type of reinforcement that complements the fabric, instead of competing with it. It’s worth noting that more stiffness isn’t always better. In particular, on smaller items where many layers come together at seams and corners, heavy interfacing can be tricky to work with.
For example, a thin, woven cotton might do well with light or medium weight interfacing to give the item a bit of added stability. If the interfacing is too heavy, the fabric will be hard to fold, turn, and press. The corners might be thick, the zipper tape may be uncomfortable to sandwich between layers, and turning the opening closed can be harder. Conversely, too light an interfacing may provide little to no extra support. Ultimately, you want to strike a balance between shape and flexibility.
As a general rule, try out your interfacing before applying it to the main pattern pieces. Take two scraps of the outer fabric of the same size and apply the interfacing to just one. After pressing, let both samples cool before comparing them. Fold, bend, and handle them the same way. You can also sew a short seam through both samples if the accessory will include multiple interfaced layers. This side-by-side comparison gives you more information than simply holding up a piece of interfacing on its own, since the interaction of the fabric and the interfacing will determine how they behave once combined.
When using fusible interfacing, it’s also important to follow the manufacturer’s directions for the product you’re using, instead of pressing with your iron in the usual manner. You don’t want to move the iron back and forth as if you were ironing a shirt, for example, as this may cause the interfacing to shift before the adhesive bonds evenly. Instead, press over small sections at a time, use a pressing cloth if recommended, and allow the fabric to cool before inspecting it. Bubbles or loose spots indicate that the bonding wasn’t complete, but too much heat may damage some fabrics or adhesive.
Keep in mind that not every piece of fabric needs to be interfaced. Sometimes, an entire accessory doesn’t need to be reinforced equally. A flap, zipper opening, strap attachment, or closure may require added stability, while a lining could be left with less or no reinforcement. Trimming excess interfacing from bulky seam allowances can also make turning and topstitching easier. When multiple interfaced pieces come together at the same corner, trim and grade the seam allowance to avoid excess bulk being trapped inside.
The best interfacing selection should make the project easier to construct, not more challenging. The fabric should retain its desired shape, yet still fold, turn, and press without excessive resistance. If the seams lie flatter, the corners shape more cleanly, and the finished accessory has sufficient body without feeling stiff, then you’ve found the right interfacing for the job.