Why Use InterfacingInterfacing is used to provide shaping, reinforcement, firming and support. Collars, cuffs, facings and plackets are the most commonly interfaced areas but far from the only uses for interfacing. Without interfacing buttonholes might rip without the interfacing to strengthen them, facings collars and cuffs would be limp and floppy without interfacing and many creative projects would never happen.
Interfacing -- Basic ChoicesInterfacing comes in various weights and sew-in or fusible form. The weight is almost always lighter than the fabric you are sewing but should always have the same care requirement. One exception is something like a baseball hat bill or fabric bowl. That interfacing is much heavier than the fabric to obtain the stiff hat bill or bowl shape.
Fuse or Sew-In InterfacingAlthough the choice ends up being a matter of preference, the fabric is the final decision. Not all fabrics can withstand the heat required for fusible interfacing. Cool Fuse� interfacing is a possibility because it adheres at a lower heat setting. Fusible interfacing also adds a bit more stiffness once it is fused than when it is standing alone. Always pre-wash your fabric to remove any finishes or chemical coatings before attempting to fuse an interfacing to it.
When you are sewing fabrics such as vinyl, velvets, beaded, sequin and treated fabrics it is best to use sew in interfacing.
Very Lightweight FabricThere are very lightweight interfacing available that will not overpower lightweight fabric but will add just enough support and shape without over powering the fabric.
TestIng Your ChoiceAt the store, lie a layer of the interfacing under a layer of your fabric choice. Test the hand and how the combination lays.
If you choose fusible, test the combination on a good size scrap before trusting it to your project. Follow the manufactures directions and allow the sample to cool and check the fuse as well as the smoothness of the fuse. Also check that none of the fusing has seeped through the fabric.