You pressed that seam. Then you pressed it again. And it still won’t stay down. If you've been quilting for any length of time, you know the frustration

Perfectly flat quilt seams are more than an aesthetic pursuit. Stubborn seams that refuse to stay flat aren't just an aesthetic problem; they throw off your block measurements, create bulk at intersections, and can make an otherwise beautiful quilt look unfinished. I’m not saying that your sewing must be perfect, but learning how to achieve flat seams will help the quilting process and also sewing garments, by the way, go smoother and will result in better-looking results. Knowing how to press seams flat starts with knowing why they're misbehaving. There's always a reason. And I can tell you already, for most of these causes, a good wool pressing mat is the fix that works before you even need starch, a clapper, or a change in technique. That’s why I’m sharing five causes and tips with you today to help you improve your sewing skills and achieve those flat seams.

The cause: Quilting cotton is naturally soft and relatively flexible. It is not flexible like stretch fabrics, of course. When you press a seam, the heat relaxes the fibers temporarily. Without something to hold the crease, as the fabric cools down, the fabric springs back.

The fix: Spray starch. Apply it to the seam before pressing. The starch stiffens the fibers enough to hold their position once the fabric cools.

Starch is a chemical used to stiffen fabrics and make them feel crisp. It has been used for centuries to stiffen cuffs, collars, and other clothing items. Starch can be applied to various types of fabrics, and quilters often use it on cotton fabric to make it stiff and easier to manage.

Quilting cotton that has been starched feels more rigid and is less prone to fraying. It also retains creases and stays smoother than non-starched fabric when ironed. So, if you have some quilt seams that keep popping up after you iron them, some spray starch can help give your fabric the rigidity it needs to stay flat.

Mary Ellen's Best Press is a popular option among quilters. You can also make a light DIY starch solution at home. A few things to watch for: starch can leave residue on your iron over time, it can burn if your iron temperature is too high, and some people find it irritating. Luckily, all of that washes out when the quilt is finished.

Many quilters have incorporated starch into their regular quilt preparation routine. They prewash their quilting fabric to preshrink it and then soak it in a starch solution before letting it dry. This creates crisp fabric that is easy to cut and piece, with minimal risk of shrinking in the future. However, there is quite a bit of debate in the quilting world about whether prewashing quilt fabric is necessary or not. I’ll let you make that choice, but you may want to check out my recent post about it: The Prewashing Debate for Quilting Fabric.

If you'd rather skip starch entirely, a wool pressing mat removes much of the need for it. Read on to learn more about a wool mat and how it handles the heat and the fibres of your quilting cotton.

This one catches a lot of people off-guard. You press the seam, the block or panel looks flat, then you turn it over and there's a faint shadow or ridge on the right side, exactly where the seam allowance sat.

The cause: The ironing board surface. A standard ironing board has very little padding, which means the seam allowance underneath your block presses into the surface and leaves a faint ridge on the right side of the fabric.

The fix: Press on a surface with give. A wool pressing mat's soft surface absorbs the seam allowance so it doesn't transfer through to the front of your block. If you've ever had a beautifully pressed block with a subtle but annoying shadow of the seam showing through, this is why, and a wool mat fixes it completely.

The cause: Steam introduces moisture into the fabric you are using. When that moisture evaporates unevenly, the seam can curl or distort, especially at the edges of a block.

The fix: A quilter's clapper. It's a flat, polished piece of wood you press firmly onto the block immediately after ironing. The clapper holds the seam in position while it cools and the wood absorbs the moisture at the same time. Some quilters swear by them; others find them one step too many. If you press on a wool mat, the mat itself already absorbs much of that moisture. This is one reason quilters who switch to wool often stop needing a clapper and they feel the clapper takes up to much space in an already cluttered sewing room.

A clapper is not the only non-electric pressing tool on the market. If you are interested in discovering other pressing tools or to learn more about the uses of different pressing tools, you can click through to this blog about non-electric pressing tools we posted on the Madam Sew Sewing and Quilting Blog.

The following image shows two half-square triangles that are pieced together. The seam is bulky where all of the points meet and the seam doesn’t lay very flat. As you can imagine, when you have a ton of half-square triangles pieced together, it can be a real challenge to get them to stay flat. See how the quilt seam below has popped up at the edges?

The cause: Every time seams meet at an intersection, you're adding layers of fabric. If you press all seams to one side, those layers stack on top of each other and create a ridge that no amount of pressing will fully flatten.

The fix: Think strategically about pressing direction before you sew. Pressing seams open at intersections distributes the bulk evenly on both sides instead of stacking it. On blocks with many points meeting like half-square triangles, flying geese, or star blocks, this makes a noticeable difference.

If you're following a pattern, it will usually tell you which way to press. If you're working without one, the general rule is to press toward the darker fabric to hide seam allowance show-through, but you can break that rule whenever it creates less bulk to press open instead.

☝️🤓Tip: If you do press seams open, keep in mind that open seams can become weak points in the quilt over time. There's no seam allowance folded to one side to reinforce the join. To compensate, make sure your quilting lines pass directly over those open seams. Stitching through the seam line during the quilting stage is one of the best ways to lock everything in place and prevent those joins from weakening with washing and use.

It’s hard to tell in the image below, but if you look closely at the seam, you’ll see a small hump/roll on the right side where the seam allowance has been pressed to that side.

One mistake that a lot of people make when they are new to quilting is that they piece blocks together with regular, all-purpose thread. Regular thread adds bulk to the seams and prevents them from laying flat when pressed. It messes with your perfect ¼” seams, too, because one side of your block might be slightly higher on the side that you pressed the seam allowance.

The cause: Standard all-purpose thread is thicker than it needs to be for quilting. That extra thickness adds bulk to every seam, which shows up as a slight roll or ridge on whichever side you've pressed the seam allowance toward.

The fix: Switch to 50wt cotton thread for piecing. It's specifically designed to be fine enough not to add bulk, but strong enough to hold a seam under the stress of quilting. Once you start using 50wt thread, the difference in how your pressed seams sit is immediately noticeable — especially in blocks with a lot of piecing.

Most persistent seam problems trace back to one of these five causes. But if you want a single change that addresses the majority of them at once, a wool pressing mat is it. The dual heat, the soft surface, and the moisture absorption take care of most of what's going wrong before you even need to think about starch, thread weight, or pressing direction.

If you haven't tried one yet, start with the standard wool pressing mat. And if you're regularly working on larger quilts, an extra large 17"×24" mat is worth the upgrade. If you want all 3 sizes, get the Madam Sew All-In-One Wool Pressing Mat Set.

Have another tip that's worked for you? Leave it in the comments. We are always happy to find other ways to get a stubborn seam to cooperate.

Happy quilting!

Cara Stromness,
Blogging for Madam Sew and Sewing Society. Cara loves sewing, quilting, and machine embroidery!